Chromium is 5 years old. WebKit is 7 years old. Do they count?
KHTML is a bit older (14 years).
KHTML is a bit older (14 years).
But WebKit development started in 2001, about 12 years ago.
Safari, using WebKit, was released in Jan 2003, over 10 years ago.
So no, WebKit doesn't count as a browser engine started within the last 10 years. ;)
5% might not sound like much, but I'd want something comparative. How much code in the 3.8 kernel is shared with what Linus released in 1991? I suspect it's similarly low, but we still recognize them as the same project.