People who attend college at an older age having doubt in themselves is something I've seen in many of my classmates, since the program I am in is designed for working adults. I'm one of the youngest in my program, and I'm a 26 y/o senior.
The sad thing is these older students are often the top of the class, yet their doubt persists. Their experience working pays off, and often their work is even paying for their classes, but doubt persists. It's even more pronounced in people who don't get the chance to attend school at all. A friend of mine, for instance, is an asylum seeker from Egypt, who works as a software engineer despite never having attended college. He's brilliant, but he still doubts his abilities compared to his college educated colleagues.
Personally, I experienced doubt in the system more than in my own abilities. I started college out of highschool majoring in plant biology. I picked biology with no real career plans, or any understanding of the implications of going into debt for an education. I really just picked based on the university, and the program was an afterthought.
After one year I began to doubt the value in a biology degree. I had no plans to go to medical school, and the thought of graduating with tens of thousands of dollars of debt and limited job prospects was terrifying.
It took me some time to choose to switch to mechanical engineering at a much less expensive (and more reputable, albeit less "fun") State university. While it was hard to give up the fun of the "college experience", I am so glad I did so. Since I've avoided the debt I otherwise would have accrued, I've had the luxury of searching for jobs which are personally fulfilling instead of going for the first opportunity that pays my debt. I've noticed my friends who have built up debt from school are much less positive about life, their careers, and the future. Seeing this has made me more thoughtful in all of life's important decisions, from where/how I live to who I spend time with.
I will say, there is a serious problem with guidance councilors encouraging all students to attend college versus working or going into a trade. 18 year olds generally have no clue what they want to do with their lives, and lack the foresight to make sound financial decisions. To encourage these students to attend expensive, often private institutions is not only irresponsible, it has the potential to ruin lives. We build up college as the be all and end all of success, when in reality it is so often just 4 years(or 5 year at many schools) of being babysat.