pretty sure flowering can theoreticly start as soon as it get out of the dirt, all i know if once u start puitting them on a 12/12 light scheduele then they start to flower just make sure there at a nice height and not too short it actually looks like one of yours could be started right this second if u wanted it to bud right quick.

BUT since you only have two plants id wait a bit longer till there a big bigger so you'll get some more bud i think it's 12 inches to 18 inches is the best time to start flowering any higher then the bottom buds dont get much light and are small post new pics soon so we can see if you could/should flower them now