which is the % of population with vote rights(above 18yo for example). Can u point me to some papers which talks about voting and ages?
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You can find minimum voting ages in Wikipedia. Most large countries use 18, except for Brazil and Indonesia.
You can find country population by age in the U.S Census Bureau International Data Base. It does not seem to use 18 as a break point, but you should be able to divide the 15-19 age group safely. This suggests slightly less than 69% of the world population is 18+ in 2011.
Voter turnout by age is harder, though you may not be interested. I would expect there to be national figures in scattered sources but not a central collection. The International Institute for Democracy and Electoral Assistance tried but only managed to list three countries.