What works for me is to come across as an OG hacker and stay up to date on whatever the hot tech is. I'm also as immature as ever and have a pretty good command of dank memes, which I think helps more than people want to admit. I'll follow the culture wherever it goes, the same as I have since I was a teenager.
I just always try to expose myself to everything so I know what's going on, from 4chan to reddit to HN to NPR to Fox News. IMO cultural awareness is the key to appearing "young", even if your physical appearance is older.