Why not try a rescue?! There are usually loads of rats desperate for new homes.
Yes - smaller rats will be younger. Most pet shops are supplied by rodent farms who care little about breeding for health or temperament, never mind the way that they keep the poor rats who have the babies, which is why so many have issues with pet stores.
In the past, I have had happy rats from pet stores. It is a totally personal decision. Now, I prefer to rescue as there are so many needing homes.