"He could use some weight." Really. D'ya think? Wow -- the owner should be giving him away, not trying to sell him.
21 is not too old generally speaking, but you don't know what you might be getting with this deal. He could have a lot of problems. Why is he so thin? Is she not able to afford his food? Does he need special food? Does he have ulcers? Does he have arthritis? What kind of behavior does he have? And, perhaps most importantly, what would you expect of him?
I wouldn't do it unless you have someone knowledgeable with you to help assess the situation. It might also be an idea, if he is close to you, to offer the owner to help with his care for a month at her place, no extra cost to her and if you are still interested, then you will take him off her hands at no extra cost to you.