Nevermind 4th gen, has dividing line for 2nd and 3rd gen been figured out?

I would say any groups debuting either on or after 2013.

Most groups in 2013 were sub-units of existing 2nd gen groups but you also had the debuts of BTS and Ladies' Code who are both considered 3rd gen.

2014 for sure as it brought Got7, Mamamoo, Red Velvet, WINNER, Lovelyz, and more.

Quote from LovenRose

EXO debuted 2012...

General opinion is 2012 but some say it started later for girl groups since close to no relevant girl groups debuted 2012 and 2013.

I agree, I feel like for ggs it was 2014

I would say 2012 for in general , Gangnam style changed the game Exo, B.A.P. and Nu'est debuted and that clearlty affected bg marketing in later years. AOA also came into the scene.