I thought (WRT mortgages) that you had to be 18 to sign a contract, but I don't know if that is just an assumption, or true in Eng&Wales but not here or whether it is right.
I've supported votes for 16 & 17 year olds since I was that age myself. So I appreciate that the referendum may have given politicians the impetus to make that change. I do think it was done cynically by the SNP as they chose not to give them the vote for the local government elections.
I don't know how the current youth parliament works, but I would support 12-16 year olds (or younger if they are already involved in the youth parliament) voting in the youth parliament on the same days as adults vote in main elections. I'd want the youth parliament to be properly consulted on any legislation which affects children, and for any proposals they bring to be taken seriously and have the potential to be taken on by the main parliament. In addition I think there should be youth reps on councils.
I think it is too easy for adults to make decisions, or just neglect policy areas which a relevant to children, I think they should have a stronger voice. I'm thinking about things like homophobic bullying in schools, the appalling state of some school toilets etc, but I'm sure they'd have a lot to say on other issues too - climate change and poverty for example.