Well, most of the time noone of any age can legally attend pub sessions, because without a Public Entertainment Licence they aren't legal in any case - but we get by, because the law gets ignored a lot of the time.

And the same principle applies for under-age people in pubs, and a lot of landlords are happy to allow sensible young people in, so long as there's no noticeable under-age drinking involved. (Because that really does threaten the licence.) And quite a lot of pubs have family rooms where there are no rules about keeping out people who are too young, and these tend to gte used for sessions often enough.

The advice about getting to festivals to build up contacts and such is good advice. One advantage folk music has over a lot of other types of music is that there isn't too many built-in ageism. People of widely different ages can share the same music and learn from each other.