In my story Silence, the main protagonists are 17 (turns 18 at the start) and 20 (turns 21 a little while later), they're university aged, so I can't go quite as low or old.
In The Paralogical Cases, Alden has just turned 21, and Ross is 20, which is a bit of a funny thing, since when they meet, Ross believes Alden to be 16-17, only for him to find out the former is in fact older than him.
I don't do really old or young main characters, since I feel too distanced from them to write them right. There are however several important secondary characters significantly older and younger (little siblings, flashbacks to when the characters were younger, parents, etc.)