If you got him from a pet store like Petco or Petsmart, he is probably wild caught and 4-6 years. Only guesses can be made, as they all grow at their own pace and also depending on their care, diet, etc. post a pic an some good guesses can be made.
Two things there is no way to even make a guess without a picture or the hatch date . Not to sound like a *** but Russian tortoises are usually are wild caught . So that being said with growth rates that very there is no way of knowing . Sorry but we usually just start from the time we get them .