It simply depends on the dog.

I have bumped dogs as young as 9-11 months ... but I will never roll one out long until it is mature (2-3 years).

To me, a short bump won't hurt a young dog, if he's ready, but I would not be judging him at that point (or stretching him out). I would just be giving quick sparring sessions, to acclimate him to his job, same as you would letting a young kid spar in boxing.

In other words, I would let my 8 year old son spar a couple of short rounds with other kids ... but I would not let him go 15 rounds with a full-grown man the size of a jockey

Jack