Any accurate descriptions of what age he was when he became a vigilante? 
 
Garth ennis says he was 26, but that seems a bit young.  
thanks

It really varies. The original Punisher did 3 tours in Vietnam, but nowadays he's written as if he served in the Arabian/Perisan Gulf conflict.  If you add up his history (on this site's wiki), then he'd probably been in mid-thirties or forties when he started, but comic character ages don't always match up with their backstories.  The MAX version of Punisher would be a bit different than the 616, but in reality the original Frank would probably be in his 70s now if based on his original Vietnam back story.
 
I'd bet that at 26 he was still in Vietnam. 3 tours would be at least 6-9 years. I can't remember if those tours were 2 or 3 years. He did some vigilante stuff while he was in Vietnam as a Marine (like fragging a few officers), but I don't know if that's what you mean or not. 
 
He also supposedly tried to go to seminary school, so that puts his entry age around 19 + 1 year (boot camp, bct, sniper school) + 9 years (3 tours) = 29 when he got back from Vietnam. Then it was a bit after that until he married and had kids before they were killed and he became the Punisher.
 
So if I had to guess, I'd bet around early to mid-30s. His daughter was at least 6-8 (I think). So if he got married within a year of returning and had the kid, then that would be at least 7-9 years after he got back from Vietnam. Unless he was married and had kids before he went to Vietnam, but I've never seen any evidence that he was married or had kids during his tours in Vietnam. He could have came back to states between his tours, got married, had a kid, etc, but I don't remember any of that.
 
I don't know if any of the books talk about what happened after he got back from Vietnam to before the killing of his family. Most Punisher stories, that I remember, are either set in Vietnam or after his family is killed.

" It really varies. The original Punisher did 3 tours in Vietnam, but nowadays he's written as if he served in the Arabian/Perisan Gulf conflict.  If you add up his history (on this site's wiki), then he'd probably been in mid-thirties or forties when he started, but comic character ages don't always match up with their backstories.  The MAX version of Punisher would be a bit different than the 616, but in reality the original Frank would probably be in his 70s now if based on his original Vietnam back story.  I'd bet that at 26 he was still in Vietnam. 3 tours would be at least 6-9 years. I can't remember if those tours were 2 or 3 years. He did some vigilante stuff while he was in Vietnam as a Marine (like fragging a few officers), but I don't know if that's what you mean or not.   He also supposedly tried to go to seminary school, so that puts his entry age around 19 + 1 year (boot camp, bct, sniper school) + 9 years (3 tours) = 29 when he got back from Vietnam. Then it was a bit after that until he married and had kids before they were killed and he became the Punisher. "

MAX Punisher and 616 Punisher are one in the same from my understanding.  

" @iLLituracy said:

MAX Punisher and 616 Punisher are one in the same from my understanding.  

 They are both Frank Castle, but they are different characters. Its like two Frank Castles in different universes. The 616 Punisher is now FrankenCastle (Frankenstein zombie thing). The MAX Punisher is just a vigilante going after mobsters, criminals, etc.  in a world without superheroes. The MAX version is more "realistic" (meaning no supers, just criminals).  The MAX Punisher doesn't interact with any other series - it's a completely self-contained/stand-alone comic series. So, they are the same guy, but they are two different stories and worlds. "