I say breed any time health clearances are done. IF a dog has the brains and structure to title, they can title any old time. If a bitch can finish her CH at 2, she can sure as hell finish it at 5. If you breed her young and she fails to finish at 5, she never had it in her in the first place! And if you breed at 2, and finish afterwards, who cares? Shes the same dog as if you had done it in the reverse.
But then you bred a dog that didn't have it in them... no?
I honestly don't care that much but I worry that it's a relative cop-out for most dogs to claim inability to title before breeding.
Sloan has been barely trialed (due to time and money), compared to those who campaign, with a brand spanking new handler and she has entry level titles and is ready to trial in mid level categories in agility, obedience, and schutzhund. She's also a world invitee for two dock diving organizations, she's been tested at barn hunts, has her CGC, has dappled in k9NW, and many other extra funs to evaluate her. That said, she's under 3 and all of this was accomplished before 2.5.
Some dogs mature later (Backups line is notorious for waiting until 2-3 to "normalize"), some handlers struggle, but far too often it begs the question:
if you can't put anything on the dog before two years old how is it you've really tested (put their drive, social stability, structure, and brain to the test in a more challenging setting than the back yard or a training field) the dog for breed worthiness? If this can be answered reasonably then, to each their own.
ETA: I should add, I don't think every dog needs to be out there and titled before 3 nor is it a slight at any dogs that wait. My question was specifically if you don't test the dog out and about before you breed how are you evaluating their breed worthiness without a personal bias and the realization that the stress of shows, etc, tend to teach us more about our dogs than a training field.