The peak for world class 100m sprinters seems to be late 20s/early 30s.
Lewis 9.86 @ 30
Bailey 9.84 @ 29
Surin 9.84 @ 32
Gay 9.69 @ 27
Gatlin 9.74 @ 33
Fredericks 9.86 @ 29
Linford Christie @ 33
At the amateur level I think a lot of people peak in their late teens/early 20's because they give up on the sport since it's hard to find coaches/people to train with and injuries. The injuries are the number one factor. To improve in sprinting you've got to build up your power. To build power you've got to build speed, which requires more volume and intensity, which increases the risk of injury. Unless you're naturally durable, which most people aren't, it requires physiotherapy and a lot of extra time spent on top of sprinting to remain injury free (weights for strength, stretching for flexibility, physio for rehab & injury prevention). The majority of people in their 20's don't have the time or resources ($$$ for physio) to commit to that, particularly when all of that commitment over the years will lead to them shaving tenths of a second of their 100m. That's just the reality of it.
