TL;DR Shingles vaccines reduces chances of dementia by 20%. Yet, most countries health systems only look at the upfront cost of ~$300 and don’t recommend for all who could benefit.

In a separate article the other factors are quoted with similar impact (listed in order of max potential magnitude) - anti depression treatment - education increases - hearing improvement - obesity reduction - low alcohol

The earlier you start the better.

That may be the case in other countries, but in the US we're generally fee-for-service, so the incentives are reversed, and we still don't give it until you're 50 (which, as someone who has had it and is under 50, annoys me to no end).

Should I get the shingles vaccine at a young age? I've had chickenpox earlier in my life.

20% of what?