post puberty boys have absolutely have overlap with over 18 adults, if for no other reason then kids can go through puberty at different ages, so some 18 years olds have voices that are just dropping while others have had their voice drop for 6+ years.
You aren't going to be able to find a set of signatures that keeps out a useful number of <18s that doesn't also flag >18s
You aren't going to be able to find a set of signatures that keeps out a useful number of <18s that doesn't also flag >18s