It can be several weeks old if thats what you want. However if you are not experienced with tortoises, it's sometimes best to start with an older tortoise thats several months or older.
Totally agree with both posts. Austin at AZ is a very caring breeder and eager to share care tips with forum members. Please do not but a tort from the big online retailers, too many horror stories and a very heartbreaking personal experience. Sure, there may be some that are ok, but too many of them just buy and quickly resell their reptiles and have no idea about their health or former husbandry practices. Buying from a forum breeder, even if you have to wait until a new clutch is ready will almost always mean you are getting a healthy tort and can follow up with questions years into the future.
There is no age limit. Just be careful who you buy from. Most people are still raising them much too dry. They are NOT animals that thrive in desert conditions as babies. They need hydration. Ask the breeder you buy from what substrate they use for the babies and how often they soak them. If they say rabbit pellets and once or twice a week, buy elsewhere. If you click on the links in my signature you will find a thread called "Hatchling Failure Syndrome" in the "Helpful Threads" link. Read that before you buy one anywhere.