Age is irrelevant. It's all about what you know and what you're willing to know that determines whether or not you're good at something. I've worked with web developers twice my age in their 50's. Some of them started programming in their late 30's. It sounds like you're proficient enough, so you'll be fine. It's all in your head my friend.

I often worry that being 25 myself is a cause for concern because non of my startup ideas have taken off, but I figure I've got my whole life to try and make a startup work.