You can flower at any time. I put them into flower the day they germinate. Catch up slowcoach, you're 6 weeks behind already
Flowering early impacts on yield. Small plants don't give the best harvests.
Best guide to when you should flower is to work out how tall you can let them grow and start them flowering at about 1/3 of that height. This is a safe formula for your bagseed. You can maybe go to 1/2 max height with some strains but unless you know what you are growing you need to leave a margin for error.