@MartinSleziak minitech was elected on SO when he was 15. On other sides, there have also been young ($< 18$) moderators, e.g.
jrg.
Doorknob is a pro-tem mod on two sites and is now 15 or 16. Age is certainly something one should think about, but we have examples that young people can make decent or good moderators (and examples that old[er] people can make terrible moderators).