Seriously I don't think their is anyway you can explain to them that they will believe you, most of those old people believe that bitcoin and cryptocurrency in general is scam, they don't believe anything good can come out of it, they only believe in fiat currency. I don't blame them because when they were still young their was nothing like bitcoin or digital currency, so it will be definitely difficult for them to understand it. Just tell them what you know about bitcoin, then forget about it, you don't have to convince them to believe in bitcoin.