Quote below from a Rootschatter who replied to this type of query:-
In 1820 the relevant act was An Act for the better preventing of clandestine Marriages. (1753) Lord Hardwicke’s Act, which says all Marriages shall be solemnized in the Presence of two or more credible Witnesses, without defining what a 'credible' witness was. It was up to who ever was officiating to decide whether they were credible or not. As long as the young person understood what they were witnessing, and could reach the register. In practice, probably about 11-12 was the youngest likely to have been asked. The advice to the clergy was these should, whenever practible be the relatives or friends of the parties, who would be able to testify afterwards to the identity of the persons from personal knowledge or recollection.
"Children should not be allowed to act as witnesses, but while the law permits girls of twelve years of age to contract matrimony, a clergyman would not consider it right to object to young persons as witnesses"
"The Marriage Law of England", James T. Hammick, published 1873.
The above was posted back in 2013 - Topic title was: Required age for marriage witnesses 19th century
