How old is a mouse that is about two inches long?

Answer 1

This really depends on the species, some are born bigger than others will ever become.

Generally the growth is a better indicator, do they have fur yet (first), have the opened their eyes yet? (about 12-13 days), are they walking around yet and do they give you a firm nibble during feeding time (14-15 days), have their ears opened up yet?

Once they grow fur, you can check out a list of your local mice species and try to find the right one, and then you can find more detailed information.

I also based this answer on what I found online and what I'm experiencing with my own found litter.

I would say though, if it doesn't have fur yet, don't count on it being old enough to survive, odds are slim.

If the eyes haven't opened yet, find a way to feed it kitten or puppy milk and it has a fighting chance.

If the eyes are open and it can eat solid foods (watch stool for warning signs), I would assume it's safe to get your hopes up.

If they passed all these stages, chances are it's an adult, as mice grow up very quickly.