Age itself does not stop anyone from investing in Bitcoin or from building more cash flow. What really matters is whether someone has discretionary income and the discipline to keep stacking without touching it for everyday bills. As you said, expenses will always be there no matter our age, so the real solution is having other income streams to support those expenses. That way, whatever goes into Bitcoin remains long term. Salaries or main income can serve as the foundation, but multiple income sources give extra room to increase discretionary income and keep the investment secure.
You must not have multiple sources of income before you can comfortably invest in Bitcoin, what you need is a good source of income that can cover your basic needs and allow you have enough discretionary income from where you will invest part into Bitcoin and part of it kept as emergency funds. This idea of having multiple sources of income have made more people spread their money and energy into things they should have avoid to focus on things that will help them just because they want to create many streams of income.
You shouldn't have multiple sources of income before you can comfortably invest in Bitcoin, this might be a made-up comment on your part because if we have multiple sources of income before investing, it creates a foundation for progress and a unique path for us. In fact when a person has money coming in from multiple sources of income, they don't have to worry about investing, meaning they can be smart investors and refrain from gambling on the future. In other words having multiple sources of income can better meet basic needs and increase our financial security because then everyone has more opportunities to earn money. So I disagree with your statement that you shouldn't have multiple sources of income before investing.
If you have a good business that is fetching you enough money, you can use part of your profits to start building your Building your Bitcoin portfolio. The focus is to have discretionary income and emergency funds to protect the investment from being sold when it was not planned.
Why only one business? If you have several businesses, it is possible to earn enough money from them too and if someone makes enough money from one business, then he has more businesses that is, he has two or three businesses, he will be able to earn a lot more money, which will result in earning much more money than he needs. So if these people have a long-term view of investing in Bitcoin, they don't have to be chaotic about their investment path because nowadays it is seen that many holders who have Bitcoin want to deposit more and more, just as those who have money want to save more and more money. Therefore those who have a large amount of savings can easily smooth out their investment journey.