Rat has passed on. How long to wait for a new one?

Question
I have a rat who passed away at a premature age. Am unsure of how she died. Found her in her cage one morning. I bought both of my female rats together and they are about a year old. How long should I wait to introduce a new rat? Is it ok to introduce a young rat?

Answer
I am sorry for your loss.  If it was sudden and she wasnt sick, it was probably her heart or a brain aneurysm. Sad but true :(  

I would get a new girl right away...maybe even two younger girls so that when they get all hyper and playful they wont annoy the older rat when she is probably outgrowing her playful ratlet stage. This way you wont have a solo rat issue like you do if you just keep getting one rat at a time. You can get young rats, 8 weeks old if you want, but be sure they cant escape the cage and introduce them slowly by keeping their cage side by side to the older rat for a few days so she realizes they are there.  Girls usually are easier to introduce to each other than boys, but not always. Some boys are a breeze while girls can be terrors. I dont want the older rat to mourn the loss of her cagemate and become sick from stress so I would get the new rat(s) right away!