Well, you don't really have much choice between tourist centric Great Wall sites and those crumbling and in poor repair where an OSHA inspector would throw up.

I guess I'd go to Mutianyu in preference to Badaling since it's much more likely that your 80 year old acquaintances have visited Badaling.

Having said that, if you take an cab early to Badaling and beat the tourist buses, you may have the site almost to yourself for a while.
Rich