All females breed at different sizes and ages. You can't know when they're ready so starting to try them when they're younger isn't a big deal imo. If they're ready, they'll go, if they aren't, then they wont.
I started pairing my female at 1250g this year. She got up to 1800ish grams. In my experience, once you start pairing it really triggers them to be pounding the rats lol.