I'm an experienced developer and I have no trouble learning things. If anything I'm faster than an inexperienced developer because where they're learning a new concept (e.g., "closures") I'm just learning the local spelling of it and flavor.
However, I do get pissy when I have to keep learning things, and never get to exploit them, always having to learn a new thing for every single project. Having started in the web arena since before "backend" and "frontend" were even necessarily separated, I've long since abandoned the frontend. More recently, I've personally been frustrated by the local conditions that have made it so that every system I deploy has required me to learn a new configuration management system. It's not so much an "oh, I'm too tired and old to learn this" but more of a professional eyeroll, "oh, geez, gotta learn some other ephemeral technology to do this job, then gotta learn some other technology that is almost all the same but for the spelling for the next project, etc."
But web frontend development really takes the cake, at least in the domains I'm familiar with.
I don't mind learning, I even enjoy it. But if you spend all your time learning and never get to exploit, you're operating at a greatly reduced efficiency. At some point you need to stop learning and actually advance some projects.