Note: I am offering this in the hope that others will be able to offer some clarification!
At some point between his 16th Birthday on the 24th March 1857 and his 21st in 1862 the Earl’s lands would have ceased to be under the control of his ‘Tutors’. He, like any other Scot of the time, could have married at the age of 14 but discovering precisely at what age he became legally recognised as controlling his inheritance is a little complicated.
Within the Age of Legal Capacity (Scotland) Act 1991 are clarifications from previous legislation as can be read here http://www.opsi.gov.uk/acts/acts1991/ukpga_19910050_en_2
It would appear that 16 was the age at which he became able to exercise control, although he would not achieve the Age of Majority, termed ‘Full Age’ until he was 21. What, if any, additional rights and responsibilities might have accrued at 18 is unclear but as far as I can tell the introduction of these is a later invention designed to ‘fill’ the five-year gap between 16 and 21?
Looking into this, which I am doing because it has obvious significance regarding the manner in which Harris was treated and the correct identification of those responsible, was a reminder of the current situation in Britain whereby, for example, a 16 year-old can sign to serve in the armed forces but will not be entitled to vote for those in control of those services until they are 18…
To return to 19thC Harris, between 1857 and 1862 the Countess of Dunmore would have passed control to her son, years which, coincidentally, were those during which the Maritime Surveys of Captains Otter and Thomas were being undertaken.