They'll learn so much more that won't transfer to as many job opportunities. For ex, say more about C++ and less about cutting edge CSS (because modern browser tech is an ocean). I suppose they might luck into other adjacent or unrelated roles with the same skills.

A few did.

  [Blake Ross] worked as an intern at Netscape at the age of 16 ... Ross became disenchanted with the browser he was working on and the direction given to it by America Online, which had recently purchased Netscape. Ross and Hyatt envisioned a smaller, easy-to-use browser that could have mass appeal, and Firefox was born from that idea ... in 2003 all of Mozilla's resources were devoted to the Firefox and Thunderbird projects. Released in November 2004, when Ross was 19, Firefox quickly grabbed market share ... with 100 million downloads in less than a year