I don’t know. I’d take explaining a complex concept to a 6 year old over a 70 year old any day. A 70 year old will probably rely on faded memories that have over time become wholly incorrect from disuse. They may give you a head start on a few sub-concepts/aspects of the concept, but in my experience working with A LOT of seniors in a former Master’s level helping profession, you’re far more likely to make faster progress with a 6 year old, and far more often then not for an elderly person, you’ll hit an insurmountable road block of understanding if it’s a concept they were never familiar with.

With effort, patience and available time, you could teach a 6 year old significant aspects of calculus. I would wager the average 70 year old whose career was never math centric, with only faded memories of minimally complex algebra, would take multiple times as long to do the same, if at all. The elasticity for new complex anything is extremely limited.