Eh. There's a three year difference (possibly two, depending on when their birthdays are) but they're both still very much children. Hell, the age gap's small enough for them to go to the same school. If Damian was a few years older (and looked it) I'd say they might be pushing it. But as they are now (or as they were before) it's fine. Especially since their friendship mostly started out as a "we're only hanging out cuz our dads are friends" type of thing.