How long does it take for a just weaned rat to get to being full grown?

I have a just weaned baby rat, a rat who is just at that level where he becomes a bigger rat (young adult), and I have a young adult who doesn't have too many weeks before he's pretty big, but how long does it take before all three are full grown like my oldest (1 year old). The reason is because they're all in a small cage because the younger two can slip through the bars and I really don't want to seperate bonded rats and I have no clue how to put plastic on wired cage bars, so I'm in a bit of a bind...