Onions (and shallots) are ready to harvest when the leaves begin to yellow. Where I live in Ontario, Canada that tends to be in July-long before flowers form. You can actually eat the onions any time while they're growing but by pulling them up you might disturb other ones you want to grow longer.
It sounds like yours are far along and ready to come up (although I'm not familiar with your growing zone or how long you've had them in the garden). I always suggest learning by doing-just dig some up and try them! If you plant to store them, leave them out to dry thoroughly, remove any debris, and keep them in a cool, dark place (preferably a hanging net bag).
They are volunteers from last year. I didn't know that I missed any until they came up in February. The leaves haven't turned yellow yet but they are budding. Do I wait until they flower and die back or is it too late?
Kelly, if your onions are sending up flower stalks they won't be good for storage and should be harvested as soon as possible. See more here:
Thanks @Douglas Hunt It's time to go pick some onions. I also have volunteer shallots and garlic but I'm not sure which is which. I'll harvest what looks like onions and try to figure out the rest. the onions are much larger but the shallots and garlic are nearly the same size.
They weren't round but long like a green onion only much larger. Whatever they are they'll get cooked up tomorrow night. I'll also be on the lookout for the remainder bolting.