J.S.
K.,
I think this is a question that you can answer... you'll know when you are ready for them to go up and down the stairs on their own. My son turned 2 the beginning of Dec., and he's been going up and down the stairs on his own for a while, but, I stay home with him (and my 3 year old and new baby), and our home is small, so I pretty much always know where he is and what he's doing.
If our house were larger, or if I couldn't always be near when he was heading up and down, I'd still have a gate up.
Jessica