I tend to color the lips of my female charaters lightly, the only cases when I don't are when:
-The character isn't suppose to look cute or beautiful (if she's attacking with a sword, ...), because she's less feminine than anothe rthing (dangerous, ...)
- The character has his mouth really open (when laughing, shouting) because I can't.
But I usually put a little light or something ^^
When using alcohol based markers, then I don't mind coloring the lips a lot because I really like the contrast ^^ For instance:
(a fanart of Flower Memories, a manga ^^)
So it depends on the way I color (Photoshop or markers),on the personnality of the character and on what I am able to do ^^
I never thought about the age of the character before coloring because I can't draw little girls, but I'll keep what you said in mind ^^