At this point in time, I don't think any introduction needs to be given to how taboo people see the act of aging-up or aging-down fictional characters with a canonical age. It's to the point where there's a tragic running joke in the DeviantArt community that it's impossible to make art of Shantae who is a property of Sega, the same company that made the Sonic games, due to the fact that "half-human half-jinn young adult wearing Arabic clothing" can be mistaken for "full human teenager". It's a highly polarizing topic and makes its way into all the rules.

This morning, I was talking with a friend (who is also on Side7, she's the one with the UPS thread) about water guns, and we were talking about how, in the US at least, toy guns have their tips colored orange (enforced by law) so that everyone knows they're for play and not real. Imagine how different the world would be if nobody came up with that. And we thought, what if, when it comes to making fan art, the same logic was applied, where the rules enforced a kind of norm where aged-up characters or aged-down characters were color-coded or given some signature trait or were required (by norm or by the rules) to wear certain clothes, like a "I had my age changed" uniform? Something that forewarns people as what to avoid or not avoid when drawing depending on their intentions? I would be surprised if something that can apply to weaponry can't apply to art.

I notice I might be having a hard time explaining the concept.

Look bradda.

Three boiled eggs and a couple handfuls o raw spinach daily is good for yas.

But remember to get them vitamin C's from capsicums or however.

Eat them rolled oats baws.

Fibre and carbohydrates.

That's how it's done.

I mean, I think the best way to go about it is to observe how the body changes as it ages and applying it to the work, instead of implementing other rules -- because your body does change a lot from being a teen to being an adult.
If you look at tutorials for drawing aging characters, you'll see the face elongates, the neck gets thicker, the shoulders get broader, the eyes get smaller, etc etc. These are just broad generalizations and might not apply to everyone but still.
Idk that's my two cents.

So, the site's official take on aging-up or aging-down a character is that it's okay if it is reasonable for the context of the image/text/etc. It is not okay to age-up or age-down a character for the sole purpose of skirting rules about character age solely to put a character in an adult-oriented scenario (sexual or otherwise).

For example, if you wanted to depict the adventures of characters that are canonically children, but later in their lives (e.g., Arnold from "Hey Arnold" as an adult, living his life), that's okay. But, if you wanted to depict canonically child characters as adults just to put them in slinky clothes or a sexual situation, that would be viewed as skirting the rules on such characters needing to be adult and would not be okay.

-- BK

DeviantArt is pretty sketchy when it comes to their policy on character ages. Even if a character canonically becomes 18 years old or older, you're still not allowed to draw suggestive artwork of them, because they go with their original age when they were first introduced (they treat them as if they're still underage), meaning they'll take the art down and issue a warning.
I tend to use indicators to reflect the age of my characters. Harkin in particular has been mistaken for a child before due to his height, and because I didn't want to risk being falsely accused of being a pro/com-shipper, I had to drop in obvious indicators that he's an adult, such as placing alcoholic beverages on shelves in his trailer, having him drink coffee, particularly strong roasts, and most importantly, his ID being visible near his computer which explicitly states his age as 22. This is the reason why I don't really have many child characters like I used to when I was a kid myself, so I at least somewhat know how it feels.