I think you are going to have to look at each breed as an individual.
IN dobes, for instance, especially depending on what things you want to pursue with your dog, 2 years old isnt feasible. You do schutzhund, your dog isn't eligible for a BH until 15 months old, and it's a rare dobe bitch able to get a III by 2 years old (rare for a bitch to get a III period, but still...). It's also rare for a dog to get upper level obedience or agility titles at 2 years old, though a CH certainly is possible. The doberman temperament test cannot be attempted for the first time until the dog is 18 months old. Heart certifications only START at 2 years old in terms of real readings - young ones are expected to be clear.
If you bred a bitch at 2, who came from a bitch bred at 2, and so forth - you could conceivably be 3 generations in before a bitch dies from cardio at 6 years old, and that's jus theartbreaking.
Some responsible dobe breeders are pushing back the ages the dogs are bred at for this particular reason.
Berlin's dam is going to be 8 years old, and her sire is over 5. If Berlin is bred, I would like to wait until she is at least 4 or 5 years old. More of a risk of complications, potentially - but I will also know more about longevity of her immediate line, I will know more of her capabilities in terms of the sports she chooses, I will have more of a health testing basis for her, and I will have had the chance to DO what I want to with her before breeding gets in the way.