Lynn may be right but like he said that is just a guess. There is really no way to know how old your tortoise is if he/she is wild caught. 4-5 inches means he/she is at least a couple years old, but there are many factors that determine growth rate, so realistically your tortoise could be anywhere from 3-20 years old. That being said, 4-6 years old is probably the most likely as Lynn said.
Sheldon is 4" and he's already an adult with plenty of growth rings and shell damage to prove it. It's impossible to know, unless your tort starts growing. If he starts growing, then he was probably less than 5 years old or so, I think.
Russians, as well as most species of tortoises, grow much slower in nature than in captivity. At 5" a female Russian is more likely to be at least 5 years old and probably more like 6-8 years old, possibly slightly older.