The retro cut-off date has been done to death on here but this is how I see it -
Old School: Pre 1990/91. Until around this time most bikes were quite similar and took inspiration from other bikes of the era - no suspension, 7 speed, U-brakes, long wheelbase, slack angles and tallish seat tubes etc.
Mid school: 1991 - 99. The introduction of the first 8 speed group, XTR, low profile cantis, Joe Murray inspired frame angles and then V brakes, the first DH specific bikes and a whole raft of different suspension designs and methods.
Modern: 2000 on. By this stage most of the advancements had been made and MTB's had developed into what we are still buying today. Hydraulic disc brakes, rear suspension became the norm, elastomers became obsolete and the 4-bar or single pivot design was now the way to go.
Nowadays we have 29ers with 10 speed and 760mm bars but the basic design has remained similar to what was available in the late 90's. In recent years we've had companies telling us we need 1 1/4 headtubes and press fit bb's, just like we had in the early 90's