I have 3 yr old (female?) and have been offered a 10-12 yr old female. Should I be concerned about the age difference? They will share a 800-900 sqft. enclosure.
Actually, yes. Tortoises don't do well if kept in pairs, no matter the sex of the tortoises. And at three years your smaller one can't be sexed yet. So you may end up with two females, or a male and a female. At any rate, you will always have a dominant and a submissive and the submissive rarely does well. If you want to get the other tortoise, section off the yard and give each one its own enclosure.
(The larger tortoise could injure or kill the smaller one)
I agree with what Yvonne said, and also 800-900 sq. ft. is too small an enclosure for an adult. Depending on the size of your 3 year old, it might be too small for that one too. Sulcatas need a lot of room to roam.