What constitutes being "old school" in the military?

If you ever took the SQT test and drove the M151

Old School is anyone who entered service 15 to 20 years earlier than you did.

Got away with saying "Nuke 'em till they glow and shoot 'em in the dark"
Submariners wore beards while underway.
Could duct tape a person naked upside down from the steam pipes and attach a vacuum cleaner to certain portion of his anatomy and when the Commanding Officer walked by he would just say "If you have time to hang around, you have time to clean" and keep going.

Grey hair

Re-up quickly, as you said it is a moving line!

I think it being "old school" is kinda set by a lot of things. I know for me I'm considered old school because I been in the same unit for 6 years and I seen alot happen and so many people and coc that I'm called old school.

There is no line. The unknown reality is that you become the soldier that brings you up. That is to say, the leadership style you experience during the impressionable time as a private or IET/BFN soldier is the leadership style that most imprints on you, and more than likely, the style you develop into. That being said, the generation of NCO's that brought up the current generation of NCO's were old school. Most junior and some senior NCO's (with less than 25 years TOS), were brought up by "Old School" NCO's and are there for poser "Old School" NCO's themselves. It has been my experience that anytime someone responds with” I guess I’m just Old School", it’s during the course of a conversation/situation in which they are unable/unwilling to adapt or imbrace a new standard or methedology. Its right up there with “That’s how we've Always done it" for me. My common reply is “What was desert storm like?" or, " No, the way we always did it was to stand shoulder to shoulder, and shoot at an enemy that’s 50 meters away with lead balls at a rate of one round per 90 seconds...since then things have changed!". Most of my peers were in middle school (like I was), when the Old School army was doing its thing.

Service between 1775 thru 1897. Anyone serving after that is not old school...

When you looked at the Women Army Corps (WAC) and fell in love.

For me serving since 1974 and finally retiring in 2015, and applying the lessons taught to me by WWII, Korean and Vietnam Vets, that were my instructors, 1SGT, MSG and 0-6; we ran in combat boots then, I am Old School.