Ds#2 is fine with stairs and has been for some time. He is 27 months old now. We took the gate off sometime before he turned two, but he was ok with stairs long before we took it off. Ds#1 was between 2 and 2.5 when we took the gate off for him.
I think it depends a little on if the kids have daily exposure to and practice with stairs. So, of course, if you have stairs in your home that get used daily, the kids are going to learn faster and be more capable on them earlier. My sister's kids, who have no stairs in their house, took much longer to get used to them. My sil, who also has no stairs, asks us to gate our stairs when they visit. Her son will be 4 in a couple of months.