Quote from failed_trad3r:
becoming millionaire by 25 is only really possible in finance and software. you have to be lucky to fall into that sector.
I suspect in most fields even moderately succesful entrepreneurship could drive one to his 1st million pretty fast. If you're talking of employees, where I live (and afaik) only guys in finance make decent coin. Sales people on commission could make very good money in several fields as well, real estate springs to mind and doesn't require much formal education.