At what age does one becomes an adult?

In most countries, a person is legally treated as an adult after his/her 18th birthday and is, considered responsible by the law for all his actions. For example, in most countries, a person is considered responsible enough to vote at the age of 18. Some cultures consider reaching physical maturity as the sign of growing-up and girls are often married at this time. In some religions, there are some ceremonies open only to boys and girls over a certain age. For instance, in Judaism, a person is allowed to read the Torah, their sacred writings, for the first time only after he/she attains a certain age.