Sorry, there is no legal parametre. The 14 rule is if looking after somebody who can not look after themself. So in your example if the 12 year old is incapable of looking after himself (highly unlikely, if of normal mentality), there are no offences.
Many, many children return from secondary school at 11 with no problems at all.
It may not be ideal, but the law is as I stated in my first thread.
It is one of the few rules regarding children that is solely based on common sense as opposed to regulatory systems. The ones that the headlines is when mum and/or dad go to Spain for a week leaving a 9 year old behind. The crime they are more often charged with is child neglect. It is based on the assumption (the crown to prove, which is important) that such a scenarion would endanger the wellbeing of the child.
Your example is fine (assuming they are both reasonably adjusted for their age)