My brother has just started basic training...
...at the age of
38 (36, I have been in Korea too long).
He will be in the army reserves if he can make it through basic. He is in good enough shape but, after years of BMX racing, triathlons and mountain biking, his knees and ankles are a little beat-up.
He had wanted to join earlier but he had to get his finances strait first (he will be taking a pretty heavy pay cut while he is in training).
Good luck Joe.