This is kind of interesting (per Quora):
The countries with younger absolute age to reach adulthood, at 15, are:
Indonesia
Iran
Myanmar
Yemen
The countries where you reach adulthood at 16 are:
Cambodia
Cuba
Kyrgyzstan
Palestine
Pakistan (only females)
Scotland (not a country)
Vietnam
You are an adult at 17 in:
North Korea
Tajikistan
Timor-Leste
You are an adult at 19 in:
Algeria
The following provinces of Canada: Nova Scotia New Brunswick British Columbia Newfoundland and Labrador Northwest Territories Yukon Nunavut
You are an adult at 20 in:
Japan
New Zealand
Paraguay
Taiwan
Thailand
You are an adult at 21 in:
Bahrain
Cameroon
CĂ´ted'Ivoire
Gabon
Grenada
Honduras
Lesotho
Madagascar
Namibia
Singapore
Swaziland
United Arab Emirates
United States - Mississippi & Puerto Rico
Zambia
In the rest of the world you are an adult at 18.