In the early days of Tumblr, David Karp didn’t tell anyone his age. He hired me when he was 19, and I worked for him for over a year before I learned that — and even then, I only knew because he let it slip in an interview. He wanted to keep it quiet for as long as he could because he knew that as soon as it got out, every story about Tumblr would just be about David’s youth.

What’s Actually Wrong with Yahoo’s Purchase of Summly – Marco.org