Our donor was 21 & unproven. At that time, we were so desperate for a cycle to work after my failed cycles & then a failed fresh DE cycle that we weren't thinking of siblings, but her young age turned out to be a plus.
We had a successful cycle with her (a FET, though) & when our minds turned to a sibling, she was still young & still available through the agency, so we have a 2nd child from another fresh cycle with her (also a FET success, interesting).
I know some people worry about unproven but also young donors due a possible immaturity/flakiness factor, but our donor was great. Even our RE raved about how mature & wonderful she was (as a person).
Good luck to you! Needless to say, we're thrilled we took a chance on a young, unproven donor.