Although Mr Sparling had been brought up by his mother's new partner Brian since age thirteen, by the time he found out about the age limit for adoption it was too late to apply.
Marrying someone who has a child doesn't make you their parent in the eyes of the law. The only way to create the legal parent-child relationship is to adopt. It's easy to see why that might be important to someone older than eighteen, and many who have step-parents - or indeed are step-parents - will understand the strength of Mr Sparling's feelings.
But it's also important to consider this in the light of the purpose of adoption. The law is clear on this. A court asked to make an adoption order must consider, above all else, "the need to safeguard and promote the welfare of the child throughout the child's life". While obviously there are benefits to the adoptive parent(s) as well, the law is not concerned with those. Adoption exists to benefit children by providing them with a stable and protective upbringing where this has not been possible in the biological family.
To achieve this, adoption gives the adoptive parent(s) parental rights and responsibilities (PRRs) and removes them from the biological parent(s). PRRs are the legal mechanism for safeguarding a child's welfare - they confer duties to the child, and the rights that are necessary to fulfil these duties. PRRs automatically come to an end when the child reaches eighteen (in fact, all but one of them terminate earlier, at sixteen).
While people aged over eighteen might still need or want parental guidance, arguably they no longer need a parent to be legally responsible for their welfare by way of PRRs. Given that this responsibility is the purpose of adoption, then, it makes sense for eighteen to be the cut-off point so that both parts of the law are consistent. If this were to change, perhaps a different form of adoption that did not create PRRs would be needed - but if so, what would its underlying purpose be?
Adoption has other legal effects, too. However, there are other ways to achieve similar results - as Childcare Minister Maree Todd told the Scottish Parliament when discussing Mr Sparling's case. Succession is an example. If you are adopted, you acquire limited but automatic rights of inheritance from the adoptive parent(s), and lose them in relation to your biological parent(s). However, parents can ensure they make provision for a child, adopted or otherwise, by making a Will.
Similarly, adoption confers a responsibility to maintain the child financially until age 18 (age 25 if the young person remains in full time education or training). However, such a duty is also owed to a young person who has been "accepted as a child of the family" and in any event, in almost all cases this is superseded by a statutory child support liability during the period the adopted child is at school.
There's more to life than practicalities, though, and it's understandable that so much emotional significance should attach to the idea of adoption. At the moment the Scottish Government have no plans to change the law, but Mr Sparling has vowed to continue the fight - so we'll see how he gets on.