You have to distinguish things that are age dependent (generic about 20 year olds at different times) as opposed to cohort dependent (specific to people born in 2021.)
When people born in 2001 are grand(p|m)as I think they'll have a similar relationship to the cohort of 2061. That is, there will be some things 2061ers will want to share with them and other things 2061ers won't.
There is some cohort ⨯ age interaction, for instance the boomer cohort has been so large that some tv viewing slots are saturated for ads about "you might not be getting all the medicare benefits you deserve" to the point that it drives away other viewers.
I actually think for a messenger platform it's probably a good thing that even grandma is on there. The problem with having a bunch of Boomers around is of course the social media posting aspect.