Age has nothing to do with it. These animals can live well into their thirties and I've seen females in their mid to late twenties still throwing good clutches. A 7 year old female is hardly what I would call old. The weight is more concerning. Can the seller give you any details about her past? Has she bred before? Was she sick? A finicky eater? Does she not put on the weight even when she eats?
You want to rule out both genetic contribution and illness before attempting to breed with that one.