Ugh, this is definitely a fear of mine. I'm 33, and the other day I realized that I'm one of the oldest employees of the startup I work at... older than my boss and possibly older than the founders themselves. It was a strange and slightly disconcerting feeling, realizing I was now the "old" guy.
I know about the ageism in the industry, and I do worry that in 5-10 years I'll be struggling to find work. I hope I'm wrong.