There's been controversy on how to time generations. Most of the issue comes from SM GG stans trying to create arbitrary start dates based on what conveniences them. Aespa debuted in the later part of the generation, but their stans REALLY wanted that "leaders of their generation" title. So they kept saying that Aespa opened the 4th generation. This would make groups like Stray Kidz & GIDLE "3rd gen", despite being recognized as 4th gen in their debut. But looking at things objective, there would only be 1 major SM GG per generation. And precisely, SMNGG will debut in the 5th generation.
Gen 1 / 1996-2000 (4 years) + transitional period 2001-2022 (2 years)
1996 HOT <---- disbanded in 2001
1996 Sechs Kies <---- disbanded in 2020, marking the end of Gen 1 before a transitional period
1997 SES <---- disbanded in 2002
1997 Baby VOX
1998 FinKL
1999 god
Gen 2 / 2003-2007 (4 years)
2003 TVXQ
2005 SJ
2006 Big Bang
2007 WG
2007 SNSD <---- SM stans created an extension of Gen 2 because SNSD debuted late in their generation
Gen 2.5 / 2008-2012 (4 years)
2008 SHINee
2009 2NE1
2009 f(x)
2010 miss A
2011 APink
2012 EXO <---- last major group of "Gen 2.5"
Gen 3 / 2013-2017 (4 years)
2013 BTS <--- first major group of Gen 3
2014 RV
2015 TWICE
2015 Seventeen
2015 BPink
2016 NCT
2017 WannaOne
Gen 4 / 2018 - 2022 (4 years)
2018 Stray Kidz <--- first major group of Gen 4
2018 GIDLE <--- GIDLE debuted after SKZ
2019 ITZY
2019 TXT
2020 Aespa <---- SM stans trying to say Aespa "opened" the 4th generation
2021 IVE
2022 NewJeans
Gen 5 / 2023-2027 (4 years)
2023 BabyMonster
2023 SMNGG
2024 HYBENGG
2025 JYPNGG