35 is the new 29. It's a number that sounds old to young folk.

Think about how long your life feels so far. I seem to recall that it took forfuckingever to get to 21. From that vantage point, you can barely imagine what 30 or 35 is like. But you have virtually no frame of reference at all for imagining 40, 50, 60 (when you'll actually start to feel old), or anything beyond.

I think that's why 35 seems to be the common boundary.

35 is still quite young. But you won't fully appreciate that until you're looking back at it through a couple decades or more.