I was 59 and DH was 61 when out DGD was born. Old enough not to feel aged by being GP's but young enough to be around and do things with her.
I have a friend who was in her mid 30's and a relative who was in their late 60's when they became GP's.
As for your youngest DC calling cousin's aunts and uncles that is sweet.
Our DGD, who is an only child and an only GC on both sides, has got her head round the fact that her aunt and uncles are mum and dads siblings and mum and dads aunts (no uncles sadly) are her GP's siblings.
What she is getting to grips with is that I, her Granny, still have 2 aunties that I used to visit (per Covid) and now speak to and write to both in their 90's.
She'll get there, we have drawn a sort of family tree and they did Queen Victoria at school and used the tree diagram so that helped.
Family structure is a one of those things that takes time for little ones to make sense of but they get there.