Aside from ~UncleOxidant's really good reply -- to save as much as possible with a goal towards being financially independent by 50 -- there are a couple of other options which anecdotally seem to have worked well for a lot of people:

You can become an expert in a niche subject. The tricky part here is that it has to be a niche subject that pays well, with a lot of demand but not so much demand that newcomers are tempted to do the work to also become an expert in it. One example of this, that most on HN would like to pretend doesn't exist, is COBOL. COBOL programmers can command some pretty nice rates now, because nobody wants to work with COBOL but it still powers a lot of the world.

It's also a great time to start your own business. Maybe your kids are out of the house, hopefully you've become a seasoned industry veteran, you've seen other people make mistakes and you've gathered together a nice bit of savings and some excellent credit. Why not put those resources and that experience to use and start your own business? See also: https://news.ycombinator.com/item?id=18212409