I am 30, but I started my PhD when I was 26.
Donut, from my experience, it all starts to go bad at 25. Things that were once pert, start to sag. Things that were once elastic, start to wrinkle. Things that were once blonde, start to grey (in truth, my first grey hair was spotted on my 30th birthday - no joke - on the morning of my 30th, I had a huge spot on my chin and spotted my first grey hair - arrrghhh!!!). Things you once popped in your mouth and forgot about, now seem to stick to your thighs and no amount of effort on the treadmill removes them. After 25 you start to loathe Newton and his horrible gravity. So depressing.