There's really no way of telling her age. Being raised dry, which this one appears to have been raised, can stunt it's growing to be slower. Then again, it may not have slowed it at all. Just no telling.
I'm glad your adopting this little tort. It is totally possible that this tort is 4 years old and weighs 5-6 lbs. I'm caring for one for a friend who's husband was deployed to Japan for 4 years. When I got him, he was about 4 years old and weighed 5 lbs. I knew he was male only because he flashed this thing.
He had a rough beginning and was showing signs of metabolic bone disease (MBD). I am feeding him a sprinkle of powdered calcium everyday, and I have been feeding him a little mazuri each day along with a bunch of good stuff. Have you've read the care sheets for sulcatas? Anyhow, congratulations on your new addition. Btw, there may be no way you can tell if this tort is a male or female yet. I think he/she's too small to tell. If you take a pic of the tail, we may be able to properly ID the sex , maybe.
Congrats on the new tortoise and welcome to the forum. Those links that Tom posted are crucial reading. We hope you enjoy your tortoise and make sure to keep up posted on your progress.
There's not a lot of sexual differentiation at that size. Keep an eye on the plastron...a scoop is a male, & convex indicates female. That's been the way we identify our rescues, & I can usually tell when they're about 10-12". Welcome to the forum.
My new sulcata is suppose to be 3 years old but only weighed 2 pounds. I have had him for two months and he's gained a pound so far, I don't think that he had a proper diet before I got him.