Not everyone is convinced that any biomarkers of aging actually exist. Some critics doubt that there is an underlying aging process at all. Therefore, there can be no marker for it.

Instead, it's believed that as people age, one or more deleterious processes can occur that may lead to cancer or heart disease or dementia or any other disease. These processes can overlap, but there is no biological process driving all of them.

Critics also argue that aging doesn't occur at a single rate, and therefore the rate of aging can't be measured. Different body functions can change at different rates over time. A person may lose muscle strength faster than eyesight deteriorates. And there can be different reasons for the deterioration. For example, a person may lose muscle strength faster because he or she never had much strength to begin with.

These criticisms aside, there are still many researchers who believe that there is a rate at which aging takes place and that it can be measured. The goal for these scientists is to find the biomarkers and convince the doubters.