Usually by the 2nd heat (sometimes 3rd) they are old enough to breed (minimum of 1 and 1/2 years, preferably closer to 2.
It will be extremely hard to keep them separated. The only way is to be locked in separate kennels or rooms. It only takes one minute so don't rely on watching them or those training pants and belly bands for a method of birth control.
Here's a link that tells about the heat cycle and what to watch for.
http://www.debbiejensen.com/heat_cycle.html