the first thing you have to nail is to provide proof of bitcoin itself. only when they already believe, teach them slowly. because without proof parents will not believe and want to accept and even learn what bitcoin is and how to get income from bitcoin.
You can prove the existence of bitcoin, but I think it will be more difficult to convince them that you can make good money on it
Not even true.
Convincing then that you could make money on it is very easy, just show them the funds and the transaction you have and they would believe you in just seconds. Old people might not be into advance technology stuff but they are not that dumb when you have proofs to prove that Bitcoin do really exist and it could change your future by believing on its capability that beats the fiat system.
Old people aren't stupid, of course. But I guess the explanation is difficult just because, as you say, Bitcoin can change my future, your future and the future of many other people, but when will people start to feel these changes? How many years will it take when Bitcoin will be accepted in all countries? Many people, old and young, even now do not know what bitcoin is. And for old people this future is very far away, moreover, it is more difficult for old people to get used to many aspects in our time. At least in my country, many pensioners are not used to paying with a pay card in stores, they receive a pension on their cards, but they withdraw cash from cards and use them. Many retirees are stuck in the time of their youth, but times have changed a lot. Many are reluctant to accept new technologies, it is more difficult for them to get used to and understand, or maybe they do not consciously understand this, because they lived without it before and everything was fine, why do they need it now? But I certainly do not speak for all the old people. I am rather talking about the stubbornness that I myself have encountered.