I've been on the planet for nearly as long as you then! As for expanding skillsets, well it does get harder to absorb stuff the older you are but that shouldn't stop you - I'm still learning as well and have been building sites for 14+ years.
Well if you're confident with manipulating the likes of Joomla and Wordpress then stick with those, if you can bend them to do stuff then that is a marketable skill. It's also worth remembering that no-one is really a true "expert", though some folks have learned to do more things than others, even self proclaimed "experts" won't be able to do it all (at least not without playing and testing), after all it's all about finding solutions to problems.
You could look into the new generation of CMS's such as Craft and Statamic, they are probably a lot easier to learn than WP/Joomla. If you can get to grips with one of these you may find plenty of work around, ok they aren't well known right now but demand for sites using them is growing.
HTML5 is really just the latest version of HTML, albeit with a few new structural tags such as HEADER, ARTICLE, SECTION etc. Worth reading up on their use.
It's well worth starting to write your own HTML+CSS, as you've been working with WP/Joomla you shouldn't have that much of a problem picking it up. Create a test site for yourself and play - makes mistakes and learn how to fix them, that's the way to learn!