NixOS is amazing, but it’s also got a crazy learning curve. Once you grok it though, it really changes the way you configure your computer.
Fedora is always my favorite big name distro, they’re constantly pushing the envelope and adopting new features that need some stability and exposure to mature.
Honestly I think if you lean into the grey hairs it can totally work for you. I understand wanting to stop the balding (assuming it’s actually happening and not just you imagining things – see a dermatologist), but just because you have some grey hairs doesn’t mean you’ll look like an old man in a couple years. You can always dye it if you want to, but as someone who’s had some grey in my hair since my 20s, it’s really not going to age you that much on its own, and some people will think it looks great.