The normals bro, invest as much as you're willing to lose. You may not be working in your seventies, so I'll not be talking about income here but retirement benefits. You could carve out money you'll not be needing in like 5 years time or more and invest into Bitcoin. I may not recommend holding for a very long term since you're old and  age is a factor to consider when holding Bitcoin.

5 years is long-term and your investments is likely to be profitable by then and you can choose to sell off or keep holding.