My brother, i think he just turned 39, 38 at youngest, just completed the Coolangatta Gold iron man race. Its a 4-5HR life saving race. (like a triathlon, but with paddle boards and ski’s also) He competed in the opens race. Mind you, had he been in the 30 - 39yrs age group, he would have been competitive.
Then again, the guy who came 2nd overall, he too was 39!