I think you are asking a wrong question. Of course there is no age limit on investing. Even a younger one can invest in bitcoin as long as he has money or know how bitcoin works. Its not about age but the ability to invest or have knowledge on something they are interested about.
There are many reasons why someone would want to invest, and one of them is as savings for old age or maybe for the next generation
[children and their families]. So age shouldn't be the main reason why someone doesn't want to invest, so anyone can do it at any age now unless their health condition is no longer possible.
Michael J. Saylor, he is 57 years old now and he is an entrepreneur who is known for consistently investing in bitcoin until today. So I mean, it can always be done but it also takes preparation and the important point is how to secure the assets.