Reaching the age of maturity

Moving to the threshold of adulthood is what many teenagers dream of. Every adolescent dreams of attaining the age of majority and the reasons could vary. A simple reason being: on attaining the age of majority a parent’s or a guardian’s legal control over a teenager is terminated. Teenagers have always been in awe of their new found legal responsibility, because at this age they are allowed to enter into contracts, make their own decisions which are not legally binding on others, purchase and consume alcoholic beverages, procure a driving licence etc. In short, they are allowed to do what was not permitted earlier.
Every country in this world has its own set of laws and regulations. Likewise, the age of majority also differs. While a majority of the countries have struck to 18 years as the age of majority, in Uzbekistan it is as low as 14 years and in countries like Argentina, Bahrain, Cameroon, etc., it is 21 years. In Guinea and Philippines it is 21, but this has a rider that the minors are emancipated upon marriage. In Pakistan for men and women it is 18 and 16 respectively. Generally in the US it is 18 years but some states like Arkansas, Nevada, Ohio, Tennessee, Utah, Virginia, Wisconsin it is 18; or if he/she is still in high school at 18, upon graduation, whichever happens to be earlier. Under the Age of Majority Act 1875, every person domiciled in India shall attain the age of majority on completion of 18 years and not before.
What is the criterion for selecting age of majority? Does it mean that a fourteen year old in Uzbekistan is far more matured than a 21 year old in Bahrain or an 18 year old in India? If so, how do we substantiate it?