Marriageable Age in the UK

The first mention of a legal age limit being set is in 1754, following The
Hardwick Act. Subsequent legislation effectively set "full Age" at 21 years
for both sexes (the minimum age for marriage "with consent remained 12 for a
girl and 14 for a boy. This was unchanged in English Law until 1929 when the
minimum age was raised from 14 for a boy and 12 for a girl to 16 for both
sexes with parental consent (or a license from a court) or 21 (full age)
without such consent. (Until 1969 when it was lowered to 18 for both sexes.)

Remember that Roman Catholic Law held sway over the population, the King
and the Law until the Reformation in 1534. The first records of a specific
English requirement for a formal ceremony seems to be :- "There appeared to
be many marriages taking place without witness or ceremony in the 1500's.
The Council of Trent was so disturbed by this, that they decreed in 1563
that marriages should be celebrated in the presence of a priest and at least
two witnesses. Marriage took on a new role of saving men and women from
being sinful, and of procreation. Love wasn't a necessary ingredient for
marriage during this era." 
 
This is also the period (1754) from which the idea of "full age" being 
required and the concept of parental consent/legal permission being 
introduced for those below "full age". Full age was previously determined 
by the normal age when "boys" began earning a living. This also marks the 
start of "Gretna Green" and the elopements to Scotland to get married.

"The notion of marriage as a sacrament and not just a contract can be 
traced St. Paul who compared the relationship of a husband and wife to 
that of Christ and his church (Eph. v, 23-32)." 
Concern is expressed for arranged marriages, which were normal until
medieval times. The statement of Pope Nicholas I in which he declared in
866, "If the consent be lacking in a marriage, all other celebrations, even
should the union be consummated, are rendered void", shows the importance of
a couple's consent to marriage. It has remained an important part of church
teaching (RC & CoE) through the years." Hebrew and other religions had
different customs and ceremonies.

The legal requirements prior to 1563 were that the couple publicly declared
their intent to be husband and wife, and providing they were of age (14 &
12) they were "married" - although there were ceremonies, pagan and Christian,
to celebrate and mark the event there was no legal requirement to do so. One
custom involving the couple jumping over a broomstick. There were exceptions
especially among the Aristocracy who used child marriages to form alliances.

To clarify: the age at which a child in 1870 - (bearing in mind compulsory
education for 5-10 years olds was only introduced in 1880) would reach the
minimum age for marriage was 14 for a boy and 12 for a girl providing they
had either a licence (for orphans) or parental consent or the underage party 
(or parties) had not already been married (if they had been previously
married and their partner died it would be presumed that as they were given
"consent" for the first marriage they were considered "adult" thereafter).

One factor that might have made them cautious about the marriage and
appearing to "do it right" was opinion on Child Prostitution. Outside
Marriage the Law concerning the "age of consent" to sex (often referred to
as "White Slave Trade" or "child prostitution") was set at 12 for a girl
until 1875 when the House of Commons - following a lengthy campaign against
the White Slave Trade and concern that young girls were being sold to
brothels, agreed to raise the Age of Consent from 12 to 13. In Sep 1885 the
Criminal Law Amendment Act further raised the age of consent for girls from
13 to 16, and strengthened existing legislation against prostitution and
proscribed relations.

(from http://marriage.about.com/cs/generalhistory/a/marriagehistory.htm)