Should there be age-related sentencing?

In countries like Canada and Britain, there is a separate sentencing structure for minors, regardless of the crime, they have committed. There have been cases of individuals aged 16 or 17 murdering classmates, parents, partners, and even strangers. Is it fair to argue that they do not understand the full nature of the consequences of their actions at that age? If they don't, is 18 also too young to be tried as an adult? If they do, what would be a reasonable age to claim minors cannot be held to the same standard as adults for their actions? Are these laws protecting children's rights, or enabling a lack of accountability?