When to Flower?

How do I make my plant flower when im growing it outside and I have no control over the dark and light periods.

the part of your question - 'no control' - says it all
generally, you're at the mercy of the sun and the strain
however, there is one way of MAYBE tweaking nature
you could cover the plant with a garbage can or something like that - change the light dark cycle
in my opinion, this is only practical if your plant is in your backyard - or very close by

Ya u will have to cover then and let no light get to them for at least 12 hours then uncover them for 12 hours of light

What could happen if when its too old for flowering and it doesn't get it's 12/12?

well, in an inside situation, you can keep a plant in veg with the 24/0 light cycle
i don't think you'll see that in outside situation, it will begin to mature - just maybe too slow to suit you though