There's not exact set age and it does vary with breed and lineage. Typicaly most breeds aren't fully mature until they're 2, small breeds included. However as far as growing goes most small breeds stop growing after a year old.
Personaly I wouldn't consider any breed a fully mature "adult" until they're 3 years old; but as I said there's no set in stone age and it's debated by vets and breeders alike.
Most pups of small stature have a lot of their physical growing done by six months and don't need the same levels of calories, fats etc that a still physically developing (ie a larger breed) puppy needs for growth factors. Since it is more important in the long run to keep your puppy at a good weight once he's matured, many vets will recommend switching at around sexual maturity (6-8 months).