I'm not sure, but in 1850 (per the census) his wife was 36 and his oldest child was 16 (I could not make out John's age on the form), so I'm guessing he was in his late 40s, early 50s by 1861.

I found a John T. Stephenson in the 25th Alabama who died in October of 1861, but I don't know why the one I'm looking for would have enlisted over there, so I doubt he is the same guy.