By 14 -15 months at least they should be able to crawl up the stairs. I have everyone gather at the bottom of the stairs (gate at stairs) and sit down. No one moves till I tell them. I then open the gate and send the older kids up then the group that is trying to stand and hold railing to go up but is slow and then the crawler uppers and I follow right behind them. If one or sometimes two gets real slow I just scoop them up and carry them. Coming down to the playroom we do it during baby naptime or I have them in chairs in the kitchen as we go after snack or put up a playpen near the stairs and the small play yard downstairs so I can run them one to the other. The middle group I have them do the turn around and crawl down and as Sunnydays noted it is a struggle to keep them from turning forwards but there is too much chance of them leaning forward, catching a shoe and falling down the stairs so backwards it is no matter how much you scream or protest and parents are reminded of the rules and why so they can teach this way at home and not the scoot down on your bum.

The trick is to have something at the top and bottom of the stairs to dump them into and all 3 into one playpen works for the few minutes involved.