Picking A Major??

I'm at the point where I'm almost a sophomore in college and they don't want us to be declared as undecided anymore, they want us to pick a major so we can stay on track to graduate. But I'm just finding it hard to do. Being in my mid 20s I already feel like I'm late to choosing. Seeing how if i would have gone to college at 18, I could be working on a masters by now. I wish i was one of those people that just knew what i wanted to fo in life, like I know I wanted to be a dr or know I wanted to be a teacher. I wanna know, especially if you are in your mid to late 20s or even 30s or 40s and chose to go back to school. How did you choose your major?