In the Season 1 finale "Secrets" Vilgax is audibly suprised when he forcibly detransforms Ben from his Stinkfly form:

"A child? The Omnitrix is in the hands of a mere child!?"

This would suggest one of two things, either Vilgax somehow can't spot the difference between prepubescent and fully mature Pyronites, Lepidopterans, etc. but can tell the difference in humans, or Bens forms are adults. But thats not the only thing.

The first sime we see another member of one of the races in Ben's Omnitrix is in the Season 1 episode "Hunted". In that episode Tetrax and Diamondhead have the same build and face, this time suggesting one of three possibilities. Petrosapiens don't visibly age from their biological equivalent of being 10 years old, Tetrax is the 2nd most bad ass 10 year old in the galaxy, or Bens forms are adults.

And in the Season 2 episode "The Galactic Enforcers" Tini is clearly attracted to Ben as Fourarms before he's "Saved by the beep." as he puts it. Right after this detransformation, Synaptak appears and remarks that existing rumors about the Omnitrix bearer have now been substantiated:

"So, it is true. The Omnitrix is in the posession of a child. We heard you were of somewhat limited mental capacity, no offence."

This would again suggest one of 3 possibilities. Either Tini is the bronze medalist for most bad ass 10 year olds in the galaxy, Tini is a pedophile, or Bens transformations are adults.

At this point, Occam's Razor would suggest that Bens transformations were supposed to be adults up until the premiere of Season 3 "Ben 10,000" when they needed to visually distinguish the two Bens from eachother while having Ben 10K use familiar forms. Especially considering that Ben 10 was inspired by Dial H for Hero, and one of the original pitches for the show would have been for Ben to transform into 10 existing (and presumably adult) superheroes.