Investment is very risky, try to invest what you can afford to lose. As you said, if you are very old, then you may not have any source of income. Maybe you want to invest with your savings or if you get an allowance from the government, you want to invest some amount from there. But you have to remember that investment is for a long term like 7 to 10 years. You have to set a goal first and keep buying continuously until the goal is met. But you should invest with the amount of money that you do not need. Never invest with the amount that you need.
Along with investing, it is very important to create an emergency fund. Because you will need an emergency fund very much to protect your holdings during unexpected financial disasters.
People should invest with money according to their ability to lose, because even if you manage everything correctly, an investment will not guarantee success, every matter involves risk, and decisions must be made through risk management, which will reduce the risk but will never bring guarantee. This is also very applicable to Bitcoin investment, the future of Bitcoin has a very high probability, but there is no guarantee, and considering this, an investor should invest an amount of money that is not important to him.