10 years is the minimum to be any sort of meaningful really, 20 years is feasible for FIRE folks, 30 begins to be the top realistic threshold for non-FIRE folks OR the children of people who’re switched on about investing. Assuming studies take 3-5 years (mine took ~9 years due to PhD), and first jobs don’t really pay much, and the need to live and not just save, a realistic timeframe for starting more serious investing for the average person is when they turn ~30.
Personally at 42 I only really learnt anything about investing at ~37, I will make sure my kids do learn by their 18th birthdays, though.
A lot of interesting information here, even in the summary doc. I’m wondering if a) momentum (sentiment) comes out as the key factor premium, and b) if value lost it lustre after it became better recognised (page 12 here, as I didn’t request the permission of the authors 
).