We have never restricted our puppies from stairs. They would experiment and climb stairs and cry for help to get down, and eventually get so they could go up and down on their own! Our second puppy, Muggs, would climb up and down a ladder that took us on to USFS property!
We have always left it to the individual ... never really encouraged it, but didn't discourage either. Out of all of them it took Linus the longest to pluck up the courage to go up and down them.
I'd be concerned about their soft bones at a young age..... HRH doesnt do stairs ...our stairway is wide open and she totally ignores that it is there.... too much work for her lazy butt to try to climb, I think. lol
Our house is 2 levels so stairs are a must. Ellie has been able to fly up them for over a month, but has just recently realized that she can go down them just as easily. She still has moments of being unsure and I'll have to carry her down them.