I doubt this has an actual answer, but it seems very common, on this board at least, for Americans to casually refer to kids by grade level, rather than by age. It’s always intrigued (and confused, as I never know how old it means) me as to why.
In the UK, no-one would refer to their child by what year of school they were in unless they were actually talking about school. Maybe they’d answer a ‘how’s your kid doing?’ type question with ‘He’s doing OK, he’s just starting year 7 this year,’ but I’ve never heard anyone say ‘I’ve got a year 7 kid at home who loves this book.’ In fact most UK adults would have to stop and think for a second (or may even be unable) to work out the age of the child if someone did.
Do other countries refer to their offspring in casual conversation by school grade or year? Why school year rather than age? Can’t you fail a year and be a ‘4th grader’ even though you’re a year older than everyone else?