My case is diametrically opposite. I can design anything and tackle just about any discipline. And my resume shows it.
Back when I was just trying to take a break from entrepreneurship and take a job for a few years I got no callbacks at all. Finally a recruiter took pity on me and opened up.
He said I had two problems. If I was dealing with a small company, one look at my resume and their would fear that I wanted a job to learn their business and become a competitor. And, at a mid to large company, when dealing with a VP or manager making the hiring decision, my resume would make them fear I would be after their job after gaining a foothold. So, he said "Nobody is going to hire you. You have no choice but to stay an entrepreneur or lie.".
Not long after that I landed a nice contract to help get astronauts to the International Space Station. That was fun and interesting...yet I eventually ran into the realty that they were paying 20-somethings about 1/3 (or less) of what I was getting. When my work was done my contract had nowhere to go.
Fuck me.
Bias, plain and simple.
All of the above. In my experience, hiring managers want to hire someone they can have fun with outside of work so commonalities such as age are very important