Other considerations about age of breeding is the mental and physical maturity of the bitch, and her overall health -- which would be a consideration at any age. But whelping and raising a litter can take more of a toll on a young bitch if she is thin anyway or not fully grown. And some young bitches do not make the best mothers, but there is no way of telling that until you have a litter of puppies.
Females do tend to gain their adult temperaments quicker than males. A twenty month old male can still be bouncing off the walls in puppy insanity, while a twenty month old female ought to be a bit more serious (don't mention Bear, she's defective), this is in general.
Sometimes it really is timing. If your bitch will be two in April and goes into heat in March, well, six months from then is September, Whelping in November, ready to go in January. If you breed her in March, puppies are born in May, and ready to go in July -- might be a better time for socializing and potty training, etc. Might be easier on the new owners. Might be easier to sell them at that time of the year.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC