Originally Posted by zakk
do you have some studies to back this up, or is this your opinion?
There is no reason a 20-something doesn't have the mental strength to dig in for the proper training to do long distances. NCAA athletes train 20hrs/week. Even as an undergrad I dragged myself out of bed for 6am workouts on a regular basis.
Physically, things are a little different. While most skeletal growth plates are closed by age 18-21 (later for men than women), some remain open until 25. Ideally, your bones would be fully formed before you started really pounding them. An 18 yo male may very well not be skeletally mature. I can't find the source in the hundreds of pages of crap we were tested on in monday's anatomy/embryoloy test so you will have to take my lowly med student word for it.
I'm only 25 and have just started to break into longer distances. I raced for a couple years before doing a 1/2IM at 23 and a 3/4IM this past summer. Even on a very low mileage plan, my first attempt at doing a marathon (at age 21) ended in disaster because I lacked the base. After 15 months battling 6 stress fractures, I finally had to go on crutches and Fosomax (yep, osteoporosis drugs). With my swimming background it really took several years to get my legs used to the pounding and I fought off injuries for several years. I would recomend that younger athletes, especially swimmers, build a solid base before going to longer distances.
I admit, though, that I am NOT just out there to finish. I set extremely high goals and put everything into meeting them. I didn't want to do longer distances until I could put in the training it took to show up prepared to race fast.