To get big onions, yes, it is much later than recommended. You can plant some, they are cheap, they will grow. Look for live seedlings. With decent soil, you should get something to eat.

Yeah, I think it's too late unless you're sure they're not day-length sensitive. If they're the "100 onion sets" package sort they sell in stores, then it's too late to get bulbs out of them this year. You can plant them to harvest as scallions, or you can plant them in a few days or weeks or months when you finish harvesting something else and have some empty space in the garden. Then cover them with mulch when it frosts and leave them be over the winter. They'll come up next year and bulb out then (though they'll also try to go to seed, so you won't want to store them for more than a few weeks after picking... and make sure you cut off any seed heads as soon as you see them... unless you want seeds).

I believe that you are in-between the optimal planting time for onions. Its too late for the spring planting season; however you should have a fall planting time coming up in August (4 weeks before the first frost). "Granex" or "Georgia Sweets" are popular sweet onions; when grown in Vidalia, Georgia they are known as Vidalia onions but the name is trademarked to that city so when grown elsewhere they are called Georgia Sweets. :-) Here is a planting guide for Pennsylvania. You do need to find out the frost dates for your area so that you can adjust the planting times for your particular region. I've found that growing/hardiness zones only help so much; if you live on a mountaintop your planting times/growing season is different from the folks 1/2 mile away in the valley. Here is a link that might be useful: Veg planting guide - PA

I don't think it's ever too late. The onions I harvested last year which I didn't feel were big enough were just kept over the garage last year, they froze solid, and just ended up coming back strong this year when I planted them back in the soil to get bigger. Here is a link that might be useful: My onions