I've been working in technology before the www was invented (early 90's), and the majority was in tech support. In fact I dropped out of college to work full time in support. I was good at it, but I knew it wasn't what I wanted to do for the rest of my career.
I switched to web development in 2011 after picking it up on the side. I was the oldest in age in the agency I started at, but very junior in experience. Although I tried to learn quickly, and my support background helped with troubleshooting and debugging, I still felt behind everyone else. I never had a bad review, nor was criticized for my code, but I thought I should know more because of my advanced age.
Ok, I'm in my mid 40's now, but that is like 100 in dev years.
I'm now on small team, building cool things for a life sciences publication, and I'm still feeling I should know more. I'm amazed that I haven't been found out and have been able to fake it enough to keep my job. Again, only positive feedback from my peers, but that little devil on my shoulder digs in its claws and whispers in my ear that I should go back to support.
I feel the imp(-postor syndrome) especially when I'm at a conference. My programming focus is within the WordPress sphere and I've met several of the really smart people who have contributed to the codebase. There is kind of a hero worship kind of thing when you meet them, knowing that they built the platform your career relies on. It really puts me in my place.
I've talked twice at my local meetup. It was on subjects I have a decent-enough handle on, but Mr. Imp was riding shotgun the entire time, making me nervous.
It is a daily struggle to overcome my impostor syndrome, especially when the code I'm working on doesn't come naturally for me.
I hope this rambling helps, and good luck with your talk!