I'm in a pretty similar situation. I started out with PHP and JavaScript too, and didn't go to school for programming. However, I've found software development is what I'm really interested in, and I regret not spending my 20s doing that instead.
Overall, the concepts in PHP are similar to C# or Java. You're dealing with types, classes, variables, inheritance, iterables, algorithms and logic in the same way.It's just that PHP is a lot more tolerant and less precise.
I haven't tried C#, but have worked a bit with Java and I have found moving from a language as flexible as PHP to Java isn't easy. I realized that despite the similarities to languages I know, learning something like Java does require concerted effort and I can't just blunder my way into it.
I remember when I was baffled by PHP, I'm sure you do - you have to approach it the same way as you learned back then. Start out with basic tutorials, and find a simple project or two you want to write in C#. The more you use it, the better you'll get just like anything else. It seems overwhelming a bit at first, but anything can be accomplished when you take it bit by bit.
It might be a few years before you have any idea what exactly a Inversion of Control Container is or why you'd ever need one, but I don't see any reason why you can't learn on your own with books and internet resources. Of course, also Stack Overflow has a huge C# community that I'm sure would be happy to help!
That said, you're not too old to go to college if that's what you want to do. I'd love to attend a school for this myself. I've found in my other pursuits that having a good teacher show you how to do something is indispensable, and can provide much deeper understanding more quickly compared to learning on your own.