My dad is retiring (age 69.25) in march. He's physically fit and intellectually sharp and a grumpy narcissistic engineer. Me and my mum have told him he needs to pick up a hobby or social activity or volunteering or book club or something, because otherwise he will only garden, watch test matches, read the paper and go for walks, while berating my mum and everyone around for not being perfect. Sigh.
My approach will be that if he's really resistant to tell him about how staying active and mentally fit is a protective factor against dementia. He's kinda constantly worried he has dementia (unreasonably)