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.
I agree with you that investors need to have source of income before they can approach the market to buy bitcoin in the dip which is a good idea, make sure you use what you can afford to lose in bitcoin whenever you want to buy bitcoin, and it will help you to focus on a long holding that will make you to earn good income from your holding, bitcoin is different from other business you can start today and start seeing instant result because there are some bearish season you will encounter in the process of holding without be move to release your bitcoin no matter the challenge surrounding you at the moment because you have other source of income you can use to solve those challenges, but if you don't have such sources of income you will not overcome the challenges, this is the reason why is good to have source of income before going into long term holding.