Playing mk oracle for a second, I believe what he means is that you have a very long runway in front of you, and it's still early to decide you want to do whatever it is you want to do. You don't need to be late teens/early 20s to decide that you're going to learn a whole new thing. mk himself is low 40s and just started a company after a 15ish year career as a scientist. Don't let yourself get trapped by the idea that you have a pick a thing a stick with it because you're getting older.
It's not so much the picking of a career or speciality that's frustrating or upsetting. My thinking on that domain is to do whatever is at the intersection of interesting, remunerative, and doesn't foreclose on too many future options. What ails me is how I tend to outsource my self-esteem to external factors, such as the attention of girls. I'm restive. I don't have the inner peace that I see lots of older people have. It's like that peace is inextricably bound up in age and experience.