Youngest: 12.
Oldest: Some uncountable trillions o' years.

As for age, it depends on the setting. If you're going full on anime-verse style shenanigans then power is hardly correlated to age and it doesn't matter. If you're going with a classic western style fantasy story, your age should probably be anywhere between 16-32, leaning in whether you are more of a brash youngster or cautious mentor archetype, and then there's of course stories where it involves characters that have existed for many eras intermixed with characters that were comparatively born yesterday...

If time travel is included, you can say goodbye to age continuity...

At the end of the day yeah. Age is entirely dependent on setting.