The composition of blood changes dramatically as we age. We can take young blood and put it into an old person and make them younger. That suggests that what is in ourselves when we are young can keep the body young, but that we lose it with age.
Aging is not linear. There are proteins characteristic of youth that increase from the moment we are born until we reach midlife; and others associated with aging whose levels begin to rise in the last third of life. We see a big change around age 40. Then there is a stabilization, then another big change at 60, and finally the big peak at 80, which is when most of us die.
No, because we don't know what the switches are that make us change phase. And even more so, we do not know if these molecules are a reflection of the organism's aging, or are responsible for it. We are talking about thousands of different molecules. An experiment would have to be done for each of them, which is complicated.
[
Collection
]
[
|
...
]