The best investment if you are young (not only if you are young) is probably not investing all your money in assets like the stock market.
You probably don't reached the highest potential of your income.
So investing in yourself will probably have an higher effect on your income than investing in assets like stocks or fonds.
If you have higher skills and knowledge on your field, you will earn more money.
I don't say that you should not invest a little bit of your money. I build up financial resource assets since I am 18 years old.
But your biggest asset is yourself. So investing in yourself should be more important to you than investing in other assets. You should always choose investing in yourself before investing in the stock market etc.
In Brian Tracys book "No Excuses" he describes the 3%-formula. He says, that to guarantee your lifelong success, you should make a decision today to invest 3% of your income back into yourself.
So if you are making 10,000$, invest 300$ in yourself.
If you are making 50,000$, invest 1,500$ in yourself.
If you are making 100,000$ invest 3,000$ in yourself.
I think you got the point.
This is definitely something that everybody can apply, no matter how much money they earn at the moment.
Investing in yourself with the 3% rule will increase your income by far more than only 3%. Especially if you are young, your yearly income will increase at a high rate if you put in the effort.