Huh? What grandparent? And where did you get 3 years from?
I am not exactly following your argument, but to clarify the point I am making is its better to be a stellar parent for the time you have with your kids even if you see less of their life or grandkids (if they give you any and not everyone will) than to be a mediocre parent and do serious emotional damage to your kid (which btw affects how they would raise any grandkids.)
As a former educator and social service worker I’ve seen way too many mediocre or outright bad parents and I know from both lived experiences and observations on others it impacts people for generations. Not saying all first time older parents are great but a lot of the dysfunction i saw in social services was from younger parents.
I know some people typically from strong and secure and supportive loving families who met their person young and were able to marry, and have kids in their mid 20s and over a decade later are happily married and doing great parenting and just thriving .
However some people for a variety of reasons may take longer to find the right person, and are more mindful in doing that and thinking about if they really want kids, rather than just rushing through life ticking off the societally accepted milestones.
Many of the greatest parents are the ones I know who made a tough decision to walk away from shitty men at say age 30 and start again which delayed them having kids. Or men who did a lot of therapy and growth before having kids to heal from past trauma and break cycles. Or tried and built some financial stability if they don’t have the type of wider family who they can get assistance from.
Again quality over quantity.