Silbury Hill 180
Image by Meic

>average age of the original builders was well below 20 <

This fascinates me, what are your sources for saying this?

FTC

I'll find out.
I was given some graphs by the chap who runs Wiltshire Young Archaeologists for fun but there is no book title on the pages.

I've asked my friend who says "Cripes!
I can't remember, but it was a book from our local library I'll go look
and get you the reference."

Don't say archaeologists don't ever do anything for you ;-)