I didn't get as far as protected mode or keyboard input, but the MBR and characters on a screen were achievable by a teenager in the early 00s with the few online guides from osdev. I know, because I was one, and I achieved those things using nasm on a pentium and ran it on a 8086 in another room. it meant a lot of trips with a floppy disk and practising my swearing.
It was achievable long before that. The only hard part in the 80's was getting access to the datasheets, usually in the form of photocopies of photocopies.
