Of course you can eat them any time from tiny green onions to to fully mature. The plants should be mostly dry tho if you wish to cure and store them. A green onion does not store well.

Walla Walla is not a storage onion, four months maybe.

You can eat them anytime, best to eat them when you want as they grow, than trying to keep all for storage. If you got them to bulb up, you did nothing wrong, they should be fine to eat.

Is there anything wrong growing a long day onion farther south? Is there just no need or advantage to grow long days, since you can grow the short day onions easily?

Long day onions need long periods of daylight to form large bulbs. In principle, they won't get that in the south. They'll grow, but they won't mature properly. Nothing wrong with growing onions that don't mature properly. That's up to you, if that's really what you want.

If you want storage onions, they are mostly long-day. Local nurseries in the south won't stock long-day onions, because they know they're the wrong kind. I'm curious where in Southern California the OP came up with Walla Wallas. Also, if you want sweet onions, there is a large selection of short-day varieties.

Walla Walla is on the low end of long day onions. Just requires a few more hours of daylight than an intermediate day onion. It is a sweet Spanish type which I have grown in middle Georgia ((Z-8). I grow mostly short day onions in winter with harvest in May-early June which frees up space for warm weather crops. Walla Walla and other sweet Spanish types push the harvest to late July -August. That gets into hot weather and onions really suffer under cloudless skies with temps hovering around 100 degrees.

Farmerdill, why do you grow Walla Wallas in Georgia? Just curious. Do Walla Wallas offer anything that regular short-day onions don't? That would be useful info for the OP. Of course, for non-storage onions, pushing the harvest a few months later can be advantageous in having onions available. Right now, my short-days get harvested in April, and store reliably until September. But maybe you're getting your Walla Walla harvest in August to give you a supply until December?

Curiosity mostly Dan. I have grown lots of varieties of onions and seldom grow a variety more than two years. I just want to see how a particular variety will do. I am one of those people that if told I can't do something I just have to find out for myself. I like sweet Spanish onions, but not any more than a good Granex or Grano. These get harvested in May-June strung up in a shed and will keep into December. When they begin sprouting, I stick them in ground and harvest green onions for the rest of the winter.

Sweet Spanish (Golden Grande)

Thank you, Farmerdill. Your experience and insights are always enormously valuable!