No, it's against the rules to ship other than day old chicks and adults, I believe. Not sure how old is the youngest, though, other than day olds. They can be in transit for days and won't make it if their yolk is already absorbed, which happens by about 3-4 days. Personally, I won't ship any live animal through the USPS. I empathize with them too closely, I guess.
Look on their website. I have found that most local USPS employees don't really know the regulations very well. I LOVE my local folks, but I know the regs better than they do.
After the 3 day rule they must each weigh 4 ounces and also be shipped express mail there is a sticky on the traders posts dealing with the older chicks it is under the 8+ week listings Are you NPIP? Can't ship legally if your not.
I just received a USPS express package of wheatie chicks from Atlanta to SE CT. It left there @ 2PM on 6/9 and arrive at my house @ 11 AM the next day. All chicks alive and well. They ranged in age from day old to 3 days. I find the Post Office personnel on my end to be fantastic, attentive and concerned about the chicks and eggs I have received. They always call me before putting them in a truck for delivery. The only issues I haver had with broken eggs is they were packed like crap.