Getting older is one of the biggest risk factors for many serious health conditions.
Diseases like cancer, arthritis, chronic heart failure and dementia become much more common later in life.
Scientists have spent years trying to understand why aging and chronic illness are so closely connected.
A review from researchers at University College London and Queen Mary University of London offers a different way of looking at the problem.
The researchers suggest that many age-related diseases may actually develop in two separate stages over a lifetime.
The Two Phases of Aging
According to the review, the first phase happens earlier in life.
During childhood and adulthood, the body constantly deals with damage from infections, injuries, stress, inflammation and environmental exposure.
Most of the time, the body repairs the damage or keeps it under control. But not everything fully disappears.
Some biological problems may remain hidden for years without causing noticeable symptoms.
For example, excessive sun exposure during childhood could contribute to skin cancer decades later.
How Disease Appears As You Get Older
The second phase happens as the body ages.
Over time, systems that normally help control damage begin to weaken. Immune defenses become less effective, tissue repair slows down and inflammation can become harder to regulate.
As those protective systems decline, earlier hidden damage may finally begin contributing to disease.
Researchers say this may help explain why so many chronic illnesses seem to appear later in life even though the original biological changes started years earlier.
The review connects this theory to several well-known conditions.
The virus that causes shingles, for instance, can stay inactive in the body for decades after chickenpox. As immune function weakens with age, the virus may reactivate.
Old injuries may also increase the risk of osteoarthritis once joints and tissues become less resilient over time.
Certain genetic mutations may stay silent for years before later raising the risk of diseases such as cancer or fibrosis.
The Connection to Evolution
The researchers also point to evolutionary biology.
Natural selection mainly favors traits that improve survival and reproduction earlier in life. Harmful changes that appear mostly in old age are less strongly filtered out over generations.
The study makes it clear that aging is not simply a slow decline happening in real time. Instead, it may involve hidden damage from earlier life combining with age-related biological changes later on.
This two-stage model could eventually help scientists develop better ways to prevent chronic disease before symptoms appear.
