What is the youngest age of someone known to have submitted a Pull Request to Bitcoin Core?
Matt Corallo (https://github.com/TheBlueMatt) was 18 when he started contributing to Bitcoin Core (back then just called Bitcoin).
At that age, he added support for UPnP, wrote the first dynamic DNS seeder, improved the build system, introduced wallet encryption, added debian package building, and fixed various bugs.
Whit Jackson, 15 is the youngest I can find. Github does not list the ages of PR authors:)
edit: I am not sure of his exact birthday. He may have been 14 when submitting this PR for bitcoin.org: