I wouldnt think there is any age requirement to flower. I have read many grow journals where people have gone straight to flower from clone. Seen some pretty amazing SOG grows I might add.
I topped my first grow because of limited height. I dare not comment on the yield; top vs not there are so many opinions.
ya you can start flowering as soon as you want i guess, and if u do top be sure to keep it in veg for a week or two till it recovers from being chopped off, then start flowering