
As the others ahve said in most breeds it is over two years of age. In some toy breeds 18 months is prefered, because they mature quicker and being so small breeders prefer to maximise the chances of easy whelping in a supple young bitch, as they often have small litters with relative to their size, large pups.
t6here really is no hurry to breed from msot bitches, but it is a good idea for the first litter to be born before they are five, as again they start to loose their elasticity, and to some extent the maternal instinct can become dulled.