This is confused by the fact that although Junior O was classed as being in Year 7 until last week, because Year 9's have gone on transfer week, my boy is classed as being Year 8 from this week
For the avoidance of doubt, the oldest a child in Year Eight (like the idea of second form in old money!) can be is 13 on 1st Sep on entry. The youngest possible age is 13 on 31 Aug the following year. (Source: former Head of Year 8, previously second form schoolmaster)
In Scotland 7 years at primary school then move into secondary secondary school. My grandson going into 1st year of secondary (year 8 at comp?) school at age 11 going on 12.