Sibs are usually adult size by a year old but still tend to have some development left until about 2 years.
Anywhere between the 1-2 year mark would be appropriate I'd think. I plan to stick to the puppy stuff for 2 years.
Honestly though, if you just make sure to feed them good, high quality food, the version (puppy vs adult) likely won't really matter unless you have a dog with special needs.