The Scottish Government say that because 16 year olds are able to vote, marry, sign contracts and consent to medical decisions like dental procedures etc therefore signing a statutory declaration to change legal sex is in the same category.
Except it's not about just changing your legal sex is it? The vast majority of older trans people, especially transwomen, keep their genitals and don't really undergo massive surgical and hormonal treatments.
Yet among young people, especially young girls, there's a massive surge in demanding access to hormones and mastectomies and hysterectomies. Being 'trans' in this demographic isn't about simply changing your hairstyle and clothing and a few pieces of documentation. It's extremely telling that the older, mature adults are happy to keep their sexual organs intact and functioning, while the younger the person, especially if they are female, the more likely they are to push ahead with extreme body modifications. And there's growing evidence that there are underlying factors for many of them - sexual abuse, bullying, eating disorders, depression and other mental health problems, autism, and don't forget peer pressure (especially via social media).
The report commissioned by the Scottish Sentencing Council says that while all young people are not really capable of making long-term rational decisions, the presence of these sort of issues and conditions further impacts on a young individual's ability to think beyond short-term gratification and detach from highly emotional situations.
Until the Scottish government can actually explain and justify what is going on for those under 25 who are jumping on the trans bandwagon, then it's clear that many of them are extremely vulnerable and are not fully sound of mind to be able to sign a statutory declaration saying that they intend to live in their acquired gender for the rest of their lives (what does that even mean?!).