1st gen - anything pre-2000
2nd gen - 2000-2011-ish

2011-early 2012 is actually a bit of a gray area, you'll see people call groups that debuted in that period (Apink, Block B, Boyfriend, EXID, BtoB, etc.) either 2, 3 or 2.5 gen.

3rd gen - 2012-ish - 2017-ish
"Traditionally" counted from EXO's debut in April 2012, but as I already mentioned, sometimes groups that debuted weeks or even months before that are also called 3rd gen

2017 is gray area again, groups from that period are referred to as either 3rd, 3.5th or 4th gen on a whim of a speaker.

4th gen - late 2017 - ongoing. I've seen people count 4th gen either from Wannaone (August 2017) or from Stray Kids (November 2017) on, that's mid to lat