As best I can tell, 10 of them that I picked up on Saturday are about 10 days old, and the other two, that I just picked up Monday are maybe a few days younger. I'm totally guessing though. Now, I'm thinking maybe I'm rushing things a bit. They're not very old at all. These are pics I took yesterday. The two younger ones are silver-laced wyandottes - they're smaller than the other girls. The other 10 are: speckled Sussex (6), Barred rock (1); Red Cross (1), Buff Orps (2).
Hm, they do seem like they could be a little young but I think as long as the sun is out they can go out, especially since it's pretty warm
maybe you could run MHP out there though just in case? Or do what june said and keep them out for very short times. After all, momma would probably have them out from day one 

mine were in the bathroom for 3 weeks even though it was only supposed to be one night lol amd I brought them in the garage when it was -5 for 2 nights in a row. This was in i think February and they are 24 weeks now so they were plenty feathered but i worry lol
If you want to use the glass tank so you can watch them, for 3 chicks, my recommendation would be a wire dog crate. You can wire a hardware cloth skirt or even tuck cardboard around the bottom to keep them from escaping. You still have good visibility and they can see you. And you can hose it off when you're done!
