If we are looking the age of 70-80 years, there won't be any more income flow, but rather, retirement benefits (if any). At this age, investment is no much a priority, cause age is far spent, and when it comes to investing in crypto currencies, Bitcoin is one and of which is best for the long term.
Also, such aged person, might not have the fluency is going through the market patterns on how to go around it, in terms of taking profits and buying at the deep.
The bottom line for this, should be for his/her children's benefits in life, cause for him, I don't think money is much important compared to family at this point. So, whatever he does related for money, is no more for him, but for his family.
IMO
From what OP said, if a 70-year-old wants to invest in Bitcoin with no prior knowledge about Bitcoin, my advice would be for them to get properly educated about Bitcoin first. Once they have a basic understanding, it will be easier for them to start. In normal circumstances, investing in Bitcoin carries potential for profit, especially with long-term investments. The key is patience investing for the long haul and forgetting about it for a while. I believe that in a couple of months or years, the price of BTC could potentially pump more than its current price.
Here, the age of the investor should be taken into account. Since he is already 70 years old, of course, according to the conventional rules, the DCA method we talk about for hiring Bitcoin AB may not be effective because he is already 70 years old, so at that age he may not have the kind of income with which he can follow the DCA method. Again, since Bitcoin is a long-term investment, how hard can a 70-plus person make his mind for long-term investment when he knows nothing about Bitcoin.