I got this one given to me when i bought a hanging chamacereus from Ebay the other week. The root tuber is huge so i have planted it in free draining soil but have not watered it yet.the plant is 16cm across so i am assuming that it would be pretty old.

As a guess I'd say it's Mammillaria hahniana -"Old Lady Cactus" Probably need to wait for it to flower to be sure. Should give you an impressive ring of flowers when it does....

Definitely M. hahniana.
It's a decent size at 16cm... I'm impressed.

CP

They must be pretty hardy!! it was in the middle of a bed of riverstone in full sun, all day for the last ten years. It was given to the lady i got it from by an elderly lady going into a nursing home.

I have gone completely Mamm mad lately, I just bought three of those last week I really like the shape of those, and yes the flowers will be awesome, you'll have to show us when she flowers