As long as you start with good rats, there is absolutely NO problem with starting later. You might get smaller litters, but that also partially depends on the line.

When I bred pet/show rats, I ALWAYS waited until a minimum of 6 months to breed females. Many times I waited until they were 8-10 months old. For me, as a "picky stuck up snobby pet breeder", this not only allowed me to monitor the line and see how it is developing over time, but also allowed me to limit the number of litters I had, and as a result the number of babies I had to place. If I didn't already have a problem in the line or with a particular rat, I never had any problems waiting this long. I had one line that did have smaller litters (average about 8), but I had another related line that tended to have larger litters (average around 10-12) even when bred at a later age.