They could be as young as 2 days old depending on how quickly they make it through the postal sysytem. The last batch I picked up there had just arrived 5 minutes earlier from the post office. My 2 local TSC put them out as soon as they arrive in store unless there are already too many out front on the sales floor. In this case they put the older chicks on sale and the new arrivals are kept out back, but once they're gone the newbies take their place at regular price. I've never seen any more than a week and a half old as TSC gets a shipment every week, at least in my area, SWNY/NWPA.
I don't get my chicks from TSC, but the feed store I use is pretty much on the same weekly schedule. However they just put the new shipment of chicks in with the same breeds, if there are any left from the previous shipments.
I LIKE that, as I prefer to pick up the slightly older chicks. They've already made it past the early "failure to thrive" shock of shipping, and are more active and have gotten used to using the feeders and waterers. For at least a week... well, depending on what day of the week I go chick shopping. My feed store's delivery day is Thursday, and I never go until Saturday or Sunday.