You stop reinventing wheels. You know where most of the holes are and can laugh when the kids fall in them. You know the hype cycles and can spot the fads and fashions. With 10 or 20 languages behind you, code becomes separate and 'above' languages, so you can think in structures and abstract algorithms. Enjoy standing up for principles, you're the man now, and there's not much left to "look up to" (especially in this morally declining business). Not having something to prove means you can know your limits, like when to quit trying to fix someone's "poor little thing with a broken wing" that ain't ever gonna fly. You've time to read all four volumes of Knuth and really (almost) understand. Also, you're not going to win that Turing award now, so enjoy not having to try.
Things to watch out for.
Look after your eyesight and posture. Spend more time thinking about code while out walking. Shout at clouds less because you realise that dumb ideas will die of their own accord in good time. Enjoy the new toys the kids make, even if they aren't "production ready serious" - they will mature in time.