It could be 5-6 years old if it grew around three growth lines per year, and in the wild that isn't super likely. Now if it were captive bred it could easily be that big.
She has around 10-15 growth lines per scute, and even though this can't be used super super accurately to tell age, usually russian tortoises grow around one per year with natural growing conditions (if it is wild caught; this does not apply to captive bred). I have also seen this with my other female I've had for 4 years that lives in a natural cycle with brumation. She has grown 4 lines since bought. So basically if what you said was true (that she is 5-6 years old) then she would have had to grow 2-3 lines per year- to achieve 10-15 lines. That is unlikely when most of the year they spend underground and when they are awake there are years they don't even grow at all because of scarce food/ the low nutrition of the desert vegetation. Also your tortoise looks awesome!
