I disagree. I think there are more capable 15 year-olds than others and it's not a comprehensive indicator, but brain development is not complete at that age an

I disagree. I think there are more capable 15 year-olds than others and it's not a comprehensive indicator, but brain development is not complete at that age and therefore having an adequate attention span to competently time keep is arguably more difficult to secure from a 15 year old than a 25 year old.