Stairs and puppies?

Hi all,

I intend to reintroduce myself when I get the chance to upload photos of my puppy boy but I just wanted to ask for some advice.

Marshall is a shih tzu and only 11 1/2 weeks old and he has already figured out how to get up the stairs, and he's quite quick at it too We had a baby gate at the bottom of the stair case but he some how managed to squeeze through it even though he has got so much bigger in the 3 weeks he has been here!

I read somewhere that puppies shouldnt be allowed to climb stairs because of their developing joints so I intend to get a gate to replace the one we had there but wanted to check at what age he would be allowed to use them? We will be allowing him to roam the whole house when he is allowed so its just a case of when that is!

Our CKCS pup started navigating stairs at 4 months old. He liked the gentle gradient wide steps on his coastal walk so we let him do that once or twice a week. He's 6 months old now and does the big stairs on his city walk but only in one direction (i.e. once per walk). It's good exercise and we don't let him overdo it. He didn't do any stairs at 11 weeks old but back then he was so small he could squeeze through the banister slats and fall through.