It means the animal is about 8+ months. They are "full grown" in the eyes of Petsmart and have reached a size where they can inflict damage onto smaller younger rats. When male adult rats reach about 8 months they begin to become much more squishy and lazier if you will. I personally think that males are much calmer than females, but females are more outgoing.
As far as choosing a rat...Rex and naked rats are often inbreed and as a result have a weak or lower immune system. These Rex and naked rats also need lots of cuddle cups and fleece furniture to keep their body temperature up. Be sure that your cage has lots of soft warm things if you do pick one of these out.
A weaker immune system can also be seen if the rat you are looking at is smaller yet the same age as the others.
Look for a smooth coat, clear skin (VITAL in choosing Rex or naked rats), pink tail with few blemishes and marks, perked ears, clear eyes, and listen very closely for 'clicking' sounds, sneezing, wheezing, or raised backs. Take a look at the feet if you can...if they are pink and the nails are normal in length often this means this rat is truly active and cleans himself often. Dirty feet and coat could mean an injury or pain that prevents the rat from properly cleaning himself.
Beware of adopting two at one time, regardless if they are together in the tank its very possible that they will begin to be stressed and territorial in the cage you have at home. Often Petsmart will just move several males into pairs trying to find one that just looks cute and but not functional. If possible ask for a pair that has been together for a long while. These would most likely be a 'bonded pair'. Also ask "do you have any other rats I can see?".
There are often very sweet pairs in the back or smaller rooms no one sees but staff. This is because these rats have been there too long or are bonded to each other and do not mix and match like the youths will do. I find that these pets are often older but much kinder than the boys out front because during the down time the staff comes to that room and plays with them often.
Its a stress relief for the staff as well as the pets.
If you do get rats from Petsmart take them to vets IMMEDIATELY.
Petsmart rats are not seen by certified vets more than once in their lives, often they are thrown in with other rats that are infected with minor issues but eventfully these small issues will become a serious problems due to not receiving treatment.
If a single rat is infected ALL the rats at that store are guaranteed to be infected as well. This is because of Petsmart mix n match system ignores common knowledge of rat health.
When at the vets ask for a Mycoplasmosis test. This will tell you if your rat will have a permanent issue. It cost about $45 per rat however, it well worth it to know your rats will be receiving the treatment they need or the knowledge that your rat is healthy.
Often with this issue a rat has an upper respirator infection and they go onto medications and become better. However after the medications stop they hit an immediate decline in health and may die suddenly. This test is a sure fire way to tell you if your rat truly has a minor issue or a life long problem that can be treated.
Right now were going through a horror story with out little boy I rescued from a Petsmart on Monday. Hes the second rescue Ive done from Petsmart. He has an upper respiratory infection that if not watched carefully could easily kill him because of how little he is. Also, his test results should be back by tomorrow, i cross my fingers its just a respiratory infection.
GOODLUCK AND MUCH LOVE!