I've heard this said many times. Most people I know who have loved the book read it before turning 20. It's a great read, but I think there's something in the format that means it's best for bright but fertile minds.
I’d agree that it feels like it was intended for a more youthful audience, but I doubt I would have made all of the mental connections that I did without the life experience and broader knowledge of my 30s.
About the first third of the book was interesting in new ways of thinking about symbols and self-reference. After that it kept looping back around the same topics without really adding anything more. The dialogues were somewhat entertaining but I found myself wishing to cut past the rhetorical fluff and get to the point.
I've re-read it twice, once around the year 2000 an once this past year. It holds up well. This book really was a "major event" as the Pulitzer committee described it.