On Friday, the Spanish government announced it was to raise the marriage age from 14 to 16 to bring it into line with the rest of Europe.

Estonia now has the lowest marriage age in Europe with teenagers able to get hitched at 15 with parental approval.

Globally, the average legal age of marriage for boys is 17 and 16 for girls but many countries permit them, particularly girls, to marry much younger.

Several places, including the state of Massachusetts in the United States, allow girls as young as 12 to get married in "exceptional circumstances" with the consent of a judge.

But in many cases this is a leftover from an earlier age and is rarely tested. Many countries with low marriage ages do not have a particularly high number of under 18s getting married.

Other countries only permit marriage among the young for certain groups. For instance, according to the US state department's humans right report on Trindad & Tobago from 2014, though the official marital age is 18 for men and women, Muslims and Hindus have a separate Marriage Act.

Muslims are permitted to get married at 16 for men and 12 for girls and Hindus at 18 and 14. The ages for Muslim marriage are shown in the table.

Meanwhile, Chinese lovebirds have to wait the longest, with men waiting until 22 and women until 20, due to Chinese population control policy that aims to limit the number of children a couple will have.

Note: The data is for the youngest possible age it is legal to get married with parental consent. Ages for legal marriage without parental consent will be higher.

Key:

* Varies between different states or regions within country, lowest permitted age shown

** Judicial consent needed, lowest age considered shown except for countries where there is no lower limit

*** Certain religious, cultural or tribal groups are permitted to marry younger, lowest permitted age shown

**** Sharia law

^Unconfirmed

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The data only shows the age at which it is legal to get married.

Many of these countries will either turn a blind eye to underage marriage or are struggling to crack down on it for a variety of reasons. Several other countries do not criminalise child marriage outright, the marriage is just considered invalid.

Many young girls are forced to become "common law wives" before they reach the age they can be legally wed.

In countries like India, where the legal age for marriage is 18 for women and 21 for men, the person who was a minor at the time has to file a complaint to get the marriage annulled. This limits the effectivness of the legislation.

In 2013, the country had an estimated 10m women aged 20-24 who had been married before they were 18, according to US Council on Foreign Relations.

According to the campaign, Girls Not Brides, one in three women in the developing world are said to be married before they turn 18.

In Niger, where the age of legal marriage for girls is 15, 76 per cent of girls are believed to be married before their 18th birthday.

Girls Not Brides' executive director, Lakshmi Sundaram told the Independent: "A range of international treaties and conventions point to the need for setting a minimum age of marriage of 18. Most countries around the world have laws that set a minimum age of marriage.

"However even if the law exists, there are a number of reasons why they may be undermined. Pluralistic legal systems may prioritise customary or religious law over national law, exceptions may be granted upon parental consent or authorisation by the court and law enforcement officials maybe poorly trained or lack awareness of the minimum age of marriage.