It's probably safe to assume that a hatchling is between 1 and 3 months old when it arrives at a pet shop. A lot of breeders don't sell to pet shops until their other outlets are exhausted and they need the space.
I can also tell you that a good pet shop won't rely on the breeder to "establish" hatchlings, and will take a few weeks to ensure the snake's have "settled in" before allowing them to be sold.
At the shop where I work, I get hatchlings at around a month old, and then I keep them for another month before we put a price and label on them. I do this because I don't want someone having to deal with feeding issues, regurgitations, or a "sketchy" snake. I like to make sure that every snake that leaves my shop is eating regularly and voluntarily, digesting without issue, and is at least moderately easy to handle(obviously depending on the snake, this may or may not be an easy task).
I know that alot of the "bad" shops out there get their snakes in and immediately turn them around, relying on the efforts of the breeder to determine stability of the hatchling. But my shop, and a few other good shops I know, won't sell any animal that has been there less than a couple weeks to ensure the customer is buying a healthy and well-established animal.
So, back to the original question...2 months of age is a reasonable guess as to the age of a hatchling that is "For Sale". The shop *should* know about how long it has been there, and *should* be able to supply feed records if you ask for them...It isn't an exact science, but you can guess their age within a couple months...