From the GRO Scotland website:
Before 1929, Scots law followed Roman law in allowing a girl to marry at twelve years of age and a boy at fourteen, without any requirement for parental consent. However, according to one early 20th-century source*, marriage in Scotland at such young ages was in practice almost unknown. No doubt if marriages between children had become common, there would have been public pressure to raise the legal minimum age of marriage earlier than 1929. The Age of Marriage Act 1929 (applying in Scotland, England & Wales but not in Northern Ireland) made void any marriage between persons either of whom was under the age of sixteen. Sixteen remains the lower age-limit today, contained in the current legislation, the Marriage (Scotland) Act 1977. Scots law still has no requirement for parental consent. *Source: Vital registration: a manual of the law and practice concerning the registration of births, deaths and marriages. (G T Bisset-Smith. 1st edition. Edinburgh: William Green & Sons, 1902)
