What constitutes being "old school" in the military?

Ate WWII C rations
Served as a crew member on M48, & M60 tanks.
Carried and qualified with the M-3 sub machinegun
Carried an M1911A1 in a shoulder holster.......Need I say more?

Stripped a floor using a buffer, set a yellow can of Johnson floor wax on fire and dripped the hot melted wax on the floor and then buffed the floor to a shine using you t-shirt.

Wore Seafarers, chambre shirt and crackerjacks, killed your feet in boondockers, drank $0.05 schooners of Schlitz on nickel night at the Jarheads bar at Norfolk Naval Shipyard, had a cigarette ration card for $2.00 cartons at sea, drank warm Pepsi and ate peanut butter and jelly sandwiches that tasted like fuel oil in front of the boilers on 12 on 12 off...

To me wearing Fatigues with medal bottoms and issue an M-1 rifle and doing PT in full uniform.

Old school is when you hit the 10 year mark in the military. At that point you were 50% on your way to a pension. Also known as a lifer who will get 20 or more years of service

The line does move, and there is "good" Old School stuff, and "bad" Old School stuff. I'd agree with some of what has been said. We change uniform items out over the years, so it's a good signal if someone talks about something like OD's (olive drabs...my first utility uniform in 1988). In about five years, it'll be "old school" to talk about wearing "blueberries" (please, noobs). :-)

For the "bad", you can know what Old School currently is by sitting in any GMT/safety standdown that discusses what is not permitted (just got someone sent home/to jail). Look at anyone E-7 and above (enlisted side) or O-5 and above (officer side)...if they are grinning or shifting uneasily in their seat...that thing, that is currently being discussed, is old school, and if there was a camera or there were witnesses...those older folks are just hoping the pics were trashed, or the witnesses are dead or struck mute by a bolt of lightning - or their career is in jeopardy.

For the "good" Old School stuff, that's a little easier to spot but harder to explain, you know it when you see it. I'll just speak about what I know, as an officer. An O-6 with a black Eisenhower jacket...is old school. A CO who invites his/her officers to their house for social calls by way of building rapport, would be considered "old school" these days. An O4 who actually takes the time to write an actual letter to their next CO before they PCS in, would be old school. An officer who invites all their officer peers, regardless of whether they like all of them, to a wetting down and picks up the tab without whining or complaining...is old school.

My $0.02.

Sure, the details of what it looked like are well documented here, but there's more.
In a nutshell, "Old School" is what won WWII. The kind of soldier that is rare if not extinct today in western society. The word 'Sacrifice' is derived from 'Sacred," and in those 'Old School' soldiers the freedom and liberty of the world was 'sacred' and they were willing to 'sacrifice' whatever was necessary to preserve it. Today, we describe only the "elite" soldier as having these qualities. Nothing in today's general training environment remotely resembles the men of that era. Training is 'gentile, soft and tender', taking the soldiers' "feelings and comfort" into account. Technology has replaced the blood and guts reality of war. In many ways it is fortunate western society can distance itself from those horrors and madness, but it comes at a high price. Our enemies are just as ruthless and vicious as ever. While the current attacks on freedom and liberty come from or enemies exploitation of a free society creating internal turmoil, they still resort to the age old methods of infiltrating society to sow seeds of division, conflict, violence, corruption, disinformation and tyranny taught in Marxist ideology which the 'Old School' easily recognizes as such.

Anytime before you got there.

Did that and more.

It's always one generation back (Stories told when the oldest among your group were recruits.)