For the longest time, my daughter (now 19) would fly enough to get Elite (per the old guidelines of earning 35K status miles) - that would give her theoretical access to MLL and other Star Alliance Gold lounges.

But I use the word "theoretical" - most lounges have an age-restriction policy for unaccompanied children - because of the licensing regulations for alcohol, unaccompanied passengers must be of the legal drinking age limit.