I think a lot of variables go into all the fast times being run during mid/early 20's. However I don't think the times couldn't be run at early to mid 30's. Especially if you come from a western country and serious training doesn't start until early 20's.
1) Injuries. You have someone who has trained without injury for 6+ years no interruption in their training. The moment they get a big injury that sidelines them for several months loosing that massive base from all those years of training, it is difficult to climb back up after loosing that huge base. Think Solinsky here... He says his speed is gone, but is it really? or is it the fact he was running 90 MPW in high school and kept training at a extremely high level for close to 10 years without injury? It took him that long to lay that awesome base. I mean Lagat could be a genetic freak, or it could be the fact that he really hasn't had a serious injury his whole career.
Few examples here:
Farah 1500m 3:28!!!! age 30!
Nick Willis runs PB this Summer age 31 Sub 3:30 first time in career!
John Walker First man to break 3:50 mile shaved a few tenths off his 3:49 at age 30! 7 years after he broke 3:50 for the firs time!
Lagat !!
El G won double gold at 30?
I think it is absolutely possible to maintain a high level of training and fitness through your early and mid 30's, however only if you stay healthy and avoid major injuries. If sprinters can run fast into their early 30's why can't distance runners... I think weight training really comes into play here after the age of 30.... maintaining muscle mass and testosterone through weight lifting is key for older distance runners.