In reply to Benjamin95:
You can be, but the odd one won't hurt. There are some top US ultra runners your age and younger.
Plenty are mid to late 20's, most 30-45 I'd guess...
You may be more likely to get injured, but it also does affect your pace.. do you still have short distance PRs you want?
A 50 miler isnt super long though, if it's a runnable one, which one is it?
My first was 22, I got 81 miles into the WHW and quit a broken man, went back in 2004 (24) finished, but slowly got bitten, but do wish I'd have concentrated on shorter distances when I was younger...
My advice, if you do it, is build up slowly, don't go over Marathon distance/50k in training and back right off afterwards and allow a long thorough recovery.