@Twelfthman My daughter turned 17 during her freshman year at college. She applied to StrawHats and NETC with a special request to allow her to audition since she would have finished one year of college before the summer season, but would only be 17. StrawHats made an exception for her, but NETC did not. My D got cast that summer, but that was the only summer she chose to do summer stock. Otherwise, I think a 17 year old still enrolled in HS would not be seen at these audition events.
As an aside (not pertaining to summer stock), I think age 17 is a difficult age to be cast in professional theater, because they can find actors 18 and over who can portray teen roles and not have to deal with the extra issues that underage actors entail.