you can basically start a 12/12 cycle when ever you would like. younger plants can take longer to actually go into flowering once you have them on 12/12. some indicas need to veg for an alloted amount of time before flowering, but generally you can switch whenever you would like.
if you want to flower when the plant is fully "mature" (not sure if this is the best word) in veg, then keep an eye out for preflowers. this means that the plant is preparing itself for flowering. this takes a while, depending on the strain, like 1-2 months. this doenst mean that you have to flowre though, you can keep vegging pretty much indefinetely.
hope this helps