It will take her time to learn to go down the stairs, but I personally would not encourage her to go down a set of stairs at her young age due to the possibility of injuring her back.
Don't worry, she'll figure it out eventually. We didn't let our pups go down stairs too often until they were about 6 months old just to be safe though. I don't think it's their back you really have to worry about at that age, but more so their shoulders. It's a lot of impact on a growing puppy's joints and such.
We carried Mishka up and down stairs for almost a year - and then down the stairs until he was 18 months. But we have a lot of stairs in our townhouse, so it would have been easy to let him overdo it if we didn't make a rule about it. And we have baby gates to keep him from going up or down without our knowledge.
Lemmy didn't start going down stairs until about 5 months old. We carried him down for a while, the encouraged him to get at least one paw down, then 2 paws, then we'd place him on the stairs near the bottom and call him excitedly so he's leap from them and be all proud of himself.