Ask HN: How old is the internet's "skin"?

I was thinking about my skin health today (I still have some acne on my face at age 34) and I remember reading articles saying we shed so many skin cells everyday and about every 7 years, your skin has replaced itself (through this shedding process)

And I had a thought - how old is the internet's skin? Meaning - how often does x% of the websites on the internet receive a partial facelift (be it cosmetic or codebase)?

Major websites such as Facebook, Apple, Microsoft, etc... shed "skin cells" pretty often - Facebook is always implementing new features; Apple and Microsoft always have new products and layouts for major projects.

Has anyone researched this? Are there any figures for how often the majority of the internet changes? IE: sheds it's skin? There are some pretty old websites out there if you go looking for them. Do 50% of the websites out there shed their skin every couple years? Less ?