It saddens me somewhat that someone who is mid to late thirties would be deemed as "over the hill" by some airlines. I'm a trainee train driver in my mid twenties, I'm the "baby" of the course, the average age is 36 and the oldest is 51. The two oldest folks on the course (49 and 51) have consistently been getting stronger exam results than many of the younger folks. Oldest trainee I've ever heard of was 55 when he started.
Since the training is fully funded by the company and is more expensive than a fATPL plus TR, they'd have a lot to lose if these folks left early, however the company is willing to put the investment in since they reckon the life experience these trainees have outweighs the shorter career length.
I was at two airline assessment days last year and the average age was most likely mid twenties at the max. Someone well into their forties could quite easily reach a command and spend a good ten years in the left hand seat, the airline (if applicable) and trainee would get a good return on investment I reckon. That is, if the airline employed them of course.