I used them on my son from around 4 months they fitted well. Altho' when he started walking we had a bit of trouble getting a good fit but now it's no problem again.
I've used WNOS from birth with Rory, albeit on a reduced rise setting. He's now five months and I have him on the longest rise setting and it fits him well. That doesn't mean it won't fit him for long mind, as I'm still using them on Ethan at 22 months! My little Rory is just a chubster...