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.
It is true that a person does not have the opportunity to earn after the age of 70, only retirement profits are available. Since there is no opportunity to earn a certain income, the extra money can be invested in Bitcoin, excluding the necessary money. If you invest the money that you will not have to face financial problems if you lose it and be patient, you can expect results in the future. Newbies need to understand that investing in Bitcoin with patience, the right approach, and a long-term plan can yield good long-term results.