I guess it depends on what your concerns are. If you're worried about raising up a hatchling and that it might not eat, or what have you, I'd say go for the older snakes.
I personally would rather pick a hatchling most of the time as then I know how the snake was raised, that it got good nutrition, and less time for it to have picked up any diseases, etc. I also think you have more of a selection and a better chance at getting the "best stock" when selecting from hatchlings than from what's left for sale as yearlings, but that's just my view. If you are looking for a pet and you're new to corns, perhaps an older snake would be a better choice.
I'd also like to second the recommendation that you look to breeders to purchase your snake. I don't recommend pet stores for the purchase of any live animal. They have usually gone through several hands, many pet stores have questionable knowledge on care and feeding, and I've not been impressed with many (if any) of the specimens I've personally ever seen in a pet store. Health is generally a big issue and I would count yourself lucky if you do get one that is healthy. If you don't go the breeder route, I'd tend to trust a reptile shop that breeds their own over one that just has a couple corns out for sale.
My $.02 and change.