Personally, I think that Smash's target audience are Nintendo fans.
Couldn't have said it better myself...
But for the series as a whole in my opinion...
64 was rated E, which is suitable for 6 and up. The game was superb for that, but probably died down around 25. But, of course, people older then that could still take a seat with the controller and still enjoy it.
Meanwhile, Melee earned itself a T (I still think E was fine...). T is classified as 13 and up, although I started at 10-12 (can't remember). Regardless, the game was suitable for anyone as low as 8, to about 30 years old. As with 64, people older then 30 could probably still enjoy the game.
Finally, the inevitable rating of Brawl. Personally, I think it is going to get the E10 rating, meaning anyone age 10 and up. But maybe a T like its brother Melee.... either rating will still probably mean people between, say, 7-10 as the low players, and maybe 30 for the oldest. As I've said with the other two, an older player may still play and enjoy the game exceptionally.
Overall, I'm going to say the target age group for the series would be between 8-30.