How old when introducing male?

I have been breeding rats for some time now, and are starting to hold back some of the female babies for future breeders.
I wanted your opinion on when to put the male in with them to start breeding. Do you go just by weight or by how many weeks the rats are?

It's how many weeks old the rats are. But I ignore that totally, when the rats are about the same size as my breeder females, they breed.

They say 5-7 months, but seriously, I don't have time for that nonsense. At 4 weeks a female is old enough to get pregnant. At 2 months to 3 months they're large enough to breed. That's when I add them into rotation.

Edit: There are some issues with this, though. Females are still mentally teens and are friggin stupid. They never know what the heck is going on, and have to muddle through raising their first litter. Then defensive biters are fed off.

Edit 2: The first edit seemed cruel. Defensive biters have traits I don't want trained/genetically passed on to offspring (studies have shown aome memories to be genetic). And, yes, they are friggin idiots. But I'm pretty loving to my rats, even when I carry them off to their deaths. They are pets that produce food and ultimately become food

Not about age it's about size, you want your females to be at an optimal size of 250 grams and you want your male to be in a similar weight range, if they are smaller they could get beat up pretty bad.