The "Free Radical Theory" states that, with accumulated free radical damage and oxidative stress biochemical and cellular processes begin to do more "incorrect" things as aging damage accumulates.
Most free radical damage occurs during times of the most active metabolic turnover.
In the human, this would be in early puberty for males and in pre-puberty and early puberty for females.
Also during this time, we possess the most physiologic reserve.
However, as damage accumulates, our physiologic reserve becomes depleted.
Thus, a 20-year-old faced with a trauma or biochemical assault can accommodate and recover faster than an 80-year-old, whose physiologic reserve has been depleted.
The "Free Radical Theory" was originally postulated by Denham Harman.