We got our daughter her first pair of shoes at 22 months when she was finally walking confidently and she was a 3F. Now, at almost 3 years old, she's a 6F. Our son is 15 months and still in his 3.5G shoes we got a couple of months ago.
I think they can go through growth spurts with feet just as with everything else. Our daughter went from a 4 to a 5 within a couple of months, then no growth for ages, then she was a 6 last summer and no change since then. Mind you, I don't expect her feet to be big - mine are a 3.5!