I just bought two albino Cory catfish because I wanted to breed them. The only way I know how to tell if it’s a boy and a girl is by size, but they seem kind of small. How old are they normally when they’re at the petstore
It's usually not too hard once they become near an inch.
I cannot make a good estimate of how old they are at the store, because I don't know how your store imports them, and how large they are when they get there.
Females are much wider, and chubbier around the girth, while males are long and slender.
I have a couple males in my tank, along with some females.
I agree with @Fanatic about sexing them.
If you are trying to breed them, it would be best to get a large group of them. That would ensure you get both males and females and they will be much more comfortable in a group rather than just a pair.