The problem with ages is they refer to tool technology rather than building technology. While the people who built the like of Newgrange could obviously build fantastic things from stone in 3500 BCE (and in Malta earlier still <and if you listen to some people over 12000 years ago>) they still lived in wooden huts.
The 'wood age' you refer to (i.e. when mega-monuments were all wood) seems to have been concurrent with very early 'stone age' proper.
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