Well 4" is pet shope size. I'm sure you know that by law a trot most by 4" to be sold in a pet store. So that one could by 3 to 4 years old. Now if you got them from a breeder they would because of the hatch date. I have 6" and the breeder told me he or she was 1 and half. I don't if that is right because is a lot biger the then my 3 to 4 year old from the pet store.
In all honesty, it is impossible to know for sure. You can make a guess as to age, but there are many factors that go into growth, including diet, humidity, enclosure space, and overall environment.
6" at 1 1/2? Someone is pulling your leg. For a well cared for, captive bred Redfoot, they would hit 6" at around 3 years old.
It also depends on how you measure it. The most helpful measurement is called the Straight-line Carapace Length (SCL) and it is the length from the front to the back of the back shell (carapace).
One way to get this length is to butt the torts head against a barrier so it pulls its head in, then make a mark where the back end of the carapace is, and measure from the barrier to the mark. I use a caliper that goes to 6" on the smaller guys. Quick, cheap, and accurate, but only helps for a few years.