In the South (or at least my Southern family) we were taught that Master was to be used for males under 12. After that, Mister would be used.
That’s changes made to it to be PC, I am fairly certain that up until the whole “master” tied to slavery thing, the proper terminology was Master for any unmarried male, Mister for a married male.
Since the slavery connotations the use you imply is more accepted, however it is not historically accurate.