I don't think age makes any difference in webdesign as long as you know your job. Since you'll spend most of your time at a desk, it might even be more suited to you than many other more physical jobs.
It's also one of the easier professions to start working in since there aren't really any certified bodies or professional requirements to practice it and anyone can call themselves a webdesigner It's one of the banes of my life because I'm not just competing with other professionals but with the 'neighbours son' or similar who can undercut me my 80%. The client doesn't know that the complete amateur has no idea what he's doing usually until they get their god awful site and then have to hire a professional to redo it.
Are you planning to work for someone or go self employed?
Also, there is so much to know I think it really helps to decide which bits you're going to become knowledgeable in and they'll be your core skills. You simply can't be expert in everything there is to know about webdesign IMO.
Dayum, I know how that feels..... sometimes I feel like I never stop learning which can get tiring on it's own and I also just wish sometimes that someone would just teach me everything I need to know, or even tell me what the things I need to know are so I'm not continually having to work it out for myself. I don't know if I ever imagined that I'd learn a set of skills and then just be able to use them but it sure didn't work out like that. I constantly feel like I don't anything or that I'm at the beginning of some new set of skills. It can be hard to sustain.
I mostly use forums and news feeds to try and stay current but sometimes I really miss having people in an office to talk to. That's the only thing I miss though.