In deviantART, anything goes if the character (fictional or not, boy or girl) is confirmed to be 18 years and above.
However, if the character is confirmed under 18, then there are a lot of restrictions of how you want to depict the character in your artwork.
deviantART also considers underwear as "nudity".
So if you were to as much as draw a 15 year old girl with her panties flashing, it will be taken down by the moderator for "under age nudity" or "under age erotica".
Also, if a character is under aged, you can't bump up their age, like saying "this character is 18 in the picture".
deviantART requires that the character actually aged in the series they're in.
For example, it's confirmed that Nanoha from Magical Girl Lyrical Nanoha grows up to be 20+ in the 3rd season.
In Naruto however, the younger characters have only grown up to 15-16 at most.
You can't draw Hinata & Sakura in an erotic situation together and say that "they are 18 during this picture".
However, it's possible to escape moderation if the character you're using is unpopular and doesn't have any readily available official data or wiki to confirm his/her age.
deviantART will only lift a finger if the one reporting provides sufficient information and confirmation.
Not everyone who reports can be bothered enough to Google for such info, unless they report to personally piss you off.
Also, if your original character is under 18 and you don't actually reveal that anywhere, deviantART won't moderate you if the character looks old enough.
Enjoy your loophole~