Probably around 9-10 yr old, my eldest daughter was earlier, my son was later. Still cutting my nearly 10yr old who has Down syndrome and I guess I will for a long time

I would say at least 10 for fingers. It is hard to cut your fingernails on you hand that you use all the time. You don’t have the same level of coordination.
I would let them try cutting their own toenails first. If they cut them crooked or out of shape a bit others won’t see it and and the child be teased.

I guess when they have the dexterity to do it. Maybe let them start with clippers before scissors.

depends hey on the actual kid. i still do this for them all.

It depends on the kids. With my daughter she was around 10

School age, around 8. Depending on how capable they are

I still cut my 8 year olds finger nails. Its pretty hard to use scissors one handed like you need to, especially when using non-dominant hand.