It depends on the career; for some (i. e. military officer positions) it is sooner than later. For some others there are indeed physical requirements (i. e. police). I always wanted to be a professional hockey player, I have not been drafted so I had to pursue different avenues and now since I wasn't drafted in 2015 I am thinking I will not try again. Thus, there may exist unwritten age-limits in sports also. However, for most there is no limit on age and if their primary need is someone that can critically think then physical requirements should not be an issue. Having texted that, you may have to convince the entity that is hiring that your age is not an issue.
I suggest you commence by representing your age like I do; after all some have concluded that I am 18 for the 3rd go-around our Sun. Oops you better not, my Better Half just yelled maturity is everything!