Thatch said that WBs and BMs crews are from the same generation.For example if you look the top 10 guys from BM and Beast pirates you will see most of the BMs crew are in their 40s(Smoothie and Snack are in their 30s),while most of the Beast pirates are in their 20s-30s( Black (Maria Ulti,Page One and Jack are in their 20s).So you could see that the Beast pirates are like new generation yonkou crew,while the BMs are from the old era.I expect that RH pirates will also have lots of guys that are in their 30s(Lucky Roo is 35 so far) similar to the BP.
Also Shiki's main crewmates were probably even more older then those four crews,similar to Roger pirates.