The one thing about breeding sizes of corns is that there are LOTS of different opinions on the subject.
I know that many, many breeders have no difficulty breeding two year old corns if they have sufficient size to them by that age. I, myself, have some corns that will be two next year. They're over 30" and are in good weight. Assuming they continue to do well through the winter, I'll probably breed them.
I have always had this practice, and after a few years of success, I can honestly say that I've never had an eggbound female of any age. However, I've heard of more eggbinding occuring in older females that are being bred for the first time than in young females breeding for the first time. Is this a hard and fast rule? Nope, but I think the "too young until 3 years/300 grams" reply is tossed about by a lot of people as if it's a hard and fast rule, and it's not either.