@Caspianberg But surely looking after a grandchild would be keeping mind active? A 70 year old taking a 4 year old to the museum/ the park/ helping with school collection at 3pm a few afternoons after the day doing what they like, socialising, working on own non paid projects or activities isn’t going to get to the point their brain is being so unused they wither away
Did all that with my own kids and it most certainly did not keep my mind active! As well as working a demanding job, long hours, DH and I took care of ours ourselves without nursery/childcare (we worked opposite days/shifts to enable, no family or help anywhere near), and none of what you describe was mentally challenging. What it is, is just time filling but not mentally challenging. Yes, sometimes children’s behaviour in itself can be challenging in the moment, but that’s more an emotionally challenging thing that lasts minutes, it’s is not stimulating/challenging.
ETA - it’s not as though I find museums etc dull, I don’t. I love going to see historical and cultural exhibitions (usually the ones that travel globally). However, these usually restrict to kids over 12yo. I have much much younger friend who also loves going, and she has a 10yo and it is a bit frustrating as he is extremely well behaved, and would be interested in many of them but cannot come with us so we have to work around her DH’s availability to be at home when we book. Point being, not really like you could take grandkids along to a lot of this anyway.