Vanitas is certainly not an "adult" in terms of physique.
Some people may be mislead because he is certainly more "buff" and muscular in stature but in terms of physical age he's no older than 14/15 just like Ventus, Roxas and Sora. Chronologically spoken Vanitas is actually only four years old just like Roxas, Naminé and Xion are only one year old while having the physical appearance of 14/15 year olds.
Comparing him with Ventus, Roxas and Sora can also further throw people off because the latter three are somewhat overly skinny twig-boys in the first place, out of those three especially Ventus.
This is reasonable though as Vanitas' preferred method in battle is brute strength and physical power, while Ventus' primary modus operandi is agility, speed and acrobatics for which such a small and thin body frame is more suited.
A good middle ground is actually Hayner, as he's also in their immediate age range but more muscular than Ven, Roxas and Sora while being less than Vanitas. This can be seen rather good in the scene when Hayner and Roxas press their elbows together in a scene during KH 2.