Investing does not have to involve money. Investing in yourself is doing whatever it takes to make yourself happy, at peace, and productive. Basically doing away with anything that hinders your growth process.
In your 20s, you are probably in your university, which means you have this academic and social life. I honestly think this is the age where you finally get the chance to learn who you are. Since you do not have your parents or guardians who are there to watch your every move. In most cases, you are miles away from home and you become your own responsibility and you are able to decide everything for yourself as much as your parents might still be funding your stay.
Investing in yourself could be in different ways. Giving yourself time to rest and regenerate is an investment since after a good amount of rest you will be productive in most cases. Take yourself on dates and create a lot of alone time for meditation purposes. These activities actually open your mind to so many ideas. For example, when you set out time to spend alone and you do not enjoy that moment then from there you will know that you are more of a social person.
Investing in yourself does not necessarily have to involve money unless you have started earning money which is also a very good thing. You can take that chance to travel to new places and explore. It does not have to be complicated. Go to school, hang out with yourself or friends and even go out once in a while. Take life as it comes, face each day at a time. Do not miss the chance to experience your present worrying about the future.