Most recommend at least the second heat to start. Usually heats are 6-7 months apart from one another. So you're looking at about 10 months, give or take. But some zoos will do light fingering and masturbation on the first heat, but be VERY careful doing that. Usually bitches are still finding their way in the world, and it only takes one bad experience at that age to kind of make them wary of sex for the rest of their life, and also cause potential behavioral issues.
Edit: I should also specify that the general consensus is the third heat being the most ideal time to fully start having sex.