hi wm
he could have enlisted anywhere then gone to chatham barracks , i,ve had a look at service /pension records but using your time frame there isn,t a james bennett with that birth year and from kent , are you working on the birth year as it appears in the 1841 census or have you definate proof of the birth year only the early censuses are notorious for being in most cases up to 5 years out or even more in some cases .
as to his being under age it did happen but i don,t know wether it counted to there real service time or not but i have seen mention on records of under age on them i believe they may have been used as bandsmen or such till they became of age proper , no doubt someone will put me right on it as i,d like to know myself .
regards
trevor