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.
I consider myself a rebel against PC. I didn’t realize we were speaking of historical reference. I thought we were discussing etiquette. From the encyclopedia...
However, Master is still used as the written form of address for boys of some undefined age, often regarded as under 13 in formal correspondence, particularly invitations to formal affairs.