Started a small mutual fund and retirement fund when I was just starting out and still in undergrad. I did not have much and was fully self sufficient. But someone came to my job and showed us how retirement plans worked and convinced me to start one. Same with a mutual fund. I never put more than $20-$40 in each because I didn’t have much but boy did that pay off.
I purchased a small condo in the city with some of the money I put away in #1. Just sold it recently (20 years after purchasing it; lived in it for 5 years, rented it out for a profit for 15 years). I made a lot of money off that sale. More money than I’ve ever seen at once.
My spouse and I have always lived below our means. Now we’re not frugal - we go out for nice dinners, travel, have kids. We also have good jobs. But, when we purchased a house we could have afforded to get one that was $600k and instead opted for a smaller townhome in a nice neighborhood for almost half the price. Living this way has paid off more than I could have ever imagined. Both of us don’t have to work. We travel whenever we want. We could technically both stop working in our 40s/50s and probably be fine. It’s a feeling of freedom. We’ve never over-extended ourselves. When our colleagues and friends were buying expensive homes and expensive cars and extending themselves, we just didn’t do that.