It depends where you're buying from on the highstreet. I bought mainly Asda jeans and never had a problem with the appropriate size for her age. I did find Next jeans to be a little low in the rise and some cheaper makes like Primark.
Most specially made for cloth clothes are well out of our price range.
Now DD is out of nappies I have the oppostite problem though as most 12-18 and 18-24 month stuff is made to go over a nappy, all be it a smaller disposable, so they all fall off her now as she has no nappy to hold them up
So at almost 18 months she's wearing 9-12 month jeans, which all look slightly half mast on her