The human body peaks in 20's, and it's downhill from 30 onward. Most of the time things that you shrugged off in 20's will come back to haunt you in one way or another, to include overeating, overdrinking, self-poisoning, and unintentional attempted suicide. Some people aren't as lucky (like me, ate like hog and gained weight like one too) but I know plenty of people that'd down a liter of 80 proof whatever-the-f and be fine in the morning, whereas I get headaches if I have more than a couple shots worth now, regardless of how much I drown myself in water.
So in short: humans are very adaptable and very resilient, and that starts to fade at 30 and just gets worse over time unless you condition yourself (for good or bad things).