When Does Aging Begin? Over 100 Experts Debate the Origins, Nature, and Future of Aging
Some say aging is an innate process that begins the moment we are born. Others argue that aging only truly starts after the age of 20. When it comes to the mystery of "when aging begins," scientists remain deeply divided, with many differing—and equally valid—opinions.
Recently, PNAS Nexus published a study that surveyed over 100 experts attending an international aging conference in Newry, Maine. The participants included luminaries such as Steve Horvath (father of the epigenetic clock), longevity animal researcher Vera Gorbunova, and AI-powered aging research company CEO Alex Zhavoronkov, alongside numerous emerging scientists and industry leaders. Yet despite their shared expertise, their answers varied widely. As Harvard Medical School’s Vadim Gladyshev joked, “In this field, there are more theories than people.”
So what sparks flew in this grand debate over one of biology’s most fundamental questions?
How Should We Define Aging?
About 30% of respondents defined aging as a progressive decline in bodily function over time, encompassing everything from cellular degradation to a deterioration in overall health. Others described it as the gradual accumulation of harmful changes. Some viewed it as a shift in physiological state—an increase in health risks and deviation from optimal functioning—while others took a demographic approach, equating aging with a rising probability of death.
Clearly, there was no consensus. Some emphasized aging’s outward manifestations, such as decreased mobility and health; others focused on internal molecular damage; and still others highlighted metrics like disease incidence and mortality rates. Each group prioritized different aspects of the aging process.
What Causes Aging?
Theories about the causes of aging were equally diverse. Some blamed cumulative damage; others cited evolutionary constraints, systemic dysregulation, or the breakdown of repair mechanisms. A few candidly admitted they didn’t know.
Gladyshev emphasized that our definition and understanding of aging’s root cause shape how we approach anti-aging interventions. For example, in the 20–25 age range, men don’t show increased mortality, yet biological markers such as the epigenetic clock indicate rising biological age. Whether this period represents growth or decline depends entirely on what functional parameters are measured. The lack of unified metrics stems from a more fundamental issue: we don’t yet fully understand the nature or origin of aging itself.
When Does Aging Start?
Most respondents agreed aging begins early in life—but precisely when remains unclear.
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18% said aging begins during gastrulation, an embryonic stage following several rounds of cell division.
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13% claimed it starts before conception, when sperm and egg reach maturity.
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16.5% agreed with Columbia University aging researcher Alan Cohen, who believes aging begins at fertilization—when the egg and sperm merge. He rejected the pre-conception theory, noting that if your parents were older, you’d essentially start aging earlier.
Other opinions included:
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Aging begins at birth
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Aging begins at puberty
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Aging starts when physical development ends—around age 20 or shortly after
Interestingly, no one thought aging starts at 30. The various answers show that many participants linked the beginning of aging with key developmental milestones, such as:
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Fusion of sperm and egg
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Moment of fertilization (genome formation)
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Onset of organismal development
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Birth
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Puberty
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End of development
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Peak physical function
According to Gladyshev, defining aging as starting during development implies that aging and development are simultaneous processes. In this view, the sum of aging across all body systems could accurately reflect the aging of the organism as a whole.
Is Aging a Disease?
More than one-third of respondents said yes, aging is a disease.
Another 35% said no, and 29% remained neutral.
Alan Cohen disagrees with labeling aging as a disease, because that implies it should be eradicated—something many researchers are, ironically, working toward. Gladyshev described the issue as complex. While aging may not fit the strict definition of disease, many diseases are essentially manifestations of accelerated aging in specific organs or systems.
One challenge, as several noted, is the ambiguity of the term "disease" itself. Until we clarify what constitutes a disease, whether aging fits that category will remain debatable.
What Is Rejuvenation?
Views on rejuvenation were even more varied than those on aging. Most saw it as a reduction in accumulated damage.
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Seven participants equated it with a reduction in biological age
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Others described it as:
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A shift to a healthier state
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Reduced disease incidence and mortality
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Resetting aging biomarkers
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Restoring balance in the body
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Reversing aging programs
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Repairing damage
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As with aging, scientists examined rejuvenation from multiple levels: molecular, physiological, and organism-wide. Some viewed rejuvenation as the opposite of aging, while others saw it as distinct but overlapping.
Do We Need a Unified Definition of Aging?
This was the only question with a slight lean toward a unified answer. Still, nearly half of participants disagreed or were neutral. Many argued that experimental research should take precedence, and that formal definitions can come later. Some believed their understanding of aging was already sufficient to make progress. Others felt that maintaining multiple definitions might be just as beneficial as having a single one.
What Are the Most Pressing Unanswered Questions in Aging?
The top priorities mentioned were:
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Understanding the essence of aging
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Discovering how to reverse aging
Other important topics included:
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Longevity interventions
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Techniques for measuring aging
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Regeneration of embryonic and germ cells
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Mechanistic understanding of aging
Additional concerns were:
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Radical lifespan extension
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Damage repair
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Links between aging and development
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Predictability of aging
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Cross-species comparisons
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Relationships between aging and disease
Respondents also noted a disconnect between today’s biggest unanswered questions and the current direction of research.
Illustration note: Just as COVID-19 can be treated with antiviral therapies, aging may be viewed as a process of accumulating damage and functional decline—potentially reversible through damage reduction and slowing degeneration.
Final Thoughts: Is There Hope for Consensus?
Gladyshev believes the disagreements stem not from personal bias, but from the intrinsic difficulty of these questions. Designing valid experiments and statistical models is incredibly challenging. Many terms in the field are poorly defined, and differences in emphasis or interpretation lead to further confusion.
Still, he notes that some shared beliefs do exist:
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Aging is real, harmful, and biologically significant
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Aging is a process with identifiable traits and a distinct starting point
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Rejuvenation is possible, meaning aging can be slowed—and perhaps even reversed
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Chronological age and biological age are not the same
These commonalities can guide future research.
Gladyshev remains optimistic. As long as we continue to clearly define the open questions, we will foster innovation, improve study design, and uncover new insights. He expects particularly rapid advances in the development of biomarkers of aging.