I think there has been a lot of discussion about this on the internet. First and second generations seem to easy to figure out. Typically first generation (1992-2003) is the older groups like H.O.T, S.E.S, Shinhwa, g.o.d, SechsKies etc. Second generation (2004-2011) would be groups like TVXQ, Super Junior, Girls' Generation, SHINee, 4Minute, Wonder Girls, 2NE1 etc.
Third and fourth generations seems to be where people are always arguing when it ends for third and when fourth starts. Or even if we are still in third generation. I think for third generation (2012-2018) groups like EXO, Red Velvet, BTS, Got7, Twice etc. If there is a fourth generation then it would be (2019 to present) groups like Itzy, TXT, ATEEZ, Cherry Bullet etc.
I think in each of the generations you can see a decline of when groups that were popular in that period decline over the years. But the third generations idols haven't really decline at all since they are still so popular.
I would check out
these threads since it seems to very hard to define where everything is.
So then Dreamcatcher and (G)I-DLE are 3rd generation.
I assume so since I think many people agree that we are still in 3rd generation and 4th hasn't started.
: 
