Bulbing onions - those that are supposed to mature into big bulbs with dried leaves around them like the ones you buy in bags at the grocery - are not mature if the leaves are still tall and green.

You can harvest and eat them now as spring onions if you wish.

But when they are mature, the leaf stalk will be all dried up and brown and may even fall over. Sometimes, however, onion plants make a flower stalk and go to seed without maturing. They are a bit tricky.

What I recommend you do is get a good veggie gardening book suited to your region, and read up on onions.
-O