There is no age limit for breeding fish. I've bred lots of fishies whose age was in the thousands.
There is a 21-fish limit for each tank (20 if you have the seahorse in the tank). If you have a lot of fish in the tank, any babies produced by pregnant fish could be "eaten" at birth. For example, let's say that you have the Plant of Fertility, and each pregnant fish produces two babies. If you breed 10 fish at once, and they are the only fish in the tank, you will lose 9 babies, since there will be room for only 11 babies.