This is obviously anecdotal evidence, but I've met a bunch of older programmers who were very good. And this is across several specializations--all the way from developing drivers to virtualization software to enterprise Java development to compilers. I suspect that, right now, by pure competence the best programmers I've met have actually been older. (And this is ignoring my professors who tend to be exceptional as well as older.)
So really, do not worry about becoming a worse programmer. Worry perhaps that younger people will be subconsciously biased against you, sure, but this is just a bias not based on reality. That said, nobody mandates startups to be started by younger people. My dad recently worked for a startup doing some very interesting low-level work (they concentrated on using SSDs to speed up servers) that got acquired by SanDisk recently. And he doesn't have a CS degree either--he got the equivalent of a Masters in Physics in Leningrad where they didn't even have CS in his time. (I gather they barely had computers at all.) And, most importantly, I've noticed that he has become much happier after working at the startup than he was while working at a bigger company (VMWare) before.
So, ultimately, all that matters is that you are a good programmer--and I imagine you are. Perhaps it will take a bit more work for you to find a job at a startup, perhaps you will sometimes encounter bias against your age, but ultimately you should be able to do whatever you like.
So really, do not worry about becoming a worse programmer. Worry perhaps that younger people will be subconsciously biased against you, sure, but this is just a bias not based on reality. That said, nobody mandates startups to be started by younger people. My dad recently worked for a startup doing some very interesting low-level work (they concentrated on using SSDs to speed up servers) that got acquired by SanDisk recently. And he doesn't have a CS degree either--he got the equivalent of a Masters in Physics in Leningrad where they didn't even have CS in his time. (I gather they barely had computers at all.) And, most importantly, I've noticed that he has become much happier after working at the startup than he was while working at a bigger company (VMWare) before.
So, ultimately, all that matters is that you are a good programmer--and I imagine you are. Perhaps it will take a bit more work for you to find a job at a startup, perhaps you will sometimes encounter bias against your age, but ultimately you should be able to do whatever you like.