my guess is around about 5-7 years i have a 3 year old that lives in a 6ft tank and i can not stretch my hand ovr his shell now i have to use 2 hands.
him being wild would have way more growth room.
that is just my guess
For an Expansa, that just a little tacker... Expansa grow very fast under optimal conditions, though not all wild turtles grow under "optimal conditions", so I'd guess it is somewhere between 5 and 30 years old