I think I understand why age-banding would be so offensive to some people. Sometimes, children can be cruel to other children who are slightly behind in academic aspects. "Eww.. so-and-so is reading books for 5-year-olds! How dumb!", they would gossip. It is true. I even had a youngster on YouTube asking me why I want to play a Grade 2 piece when I am already in Grade 6. As if it were anything shameful

I learnt German for a couple of years. In the language school library, there were readers banded with the levels e.g. A1 - A3, B1 to B4, C1-C2 . The bands where actually attached by the librarians and teachers as a guide to the standard required to read the vocabulary, and has nothing to do with content, since everything was meant for teenage to mature foreign language learners. What I found very useful was the reference statement at the back of the book (printed by publisher), which wrote something like "
For learners who have picked up between 200 and 500 German words", and so on. The covers did not say "ELEMENTARY" or "PRE-SCHOOL". I think that was a very positive way of expressing the difficulty levels.
The number of words learnt was a good reference for me. At the beginning, I was reading books below the level of my classes, but nobody could tell, unless they looked very closely. Then I progressed a little within the band, i.e. from the category of below 500 words to below 800 words, and so on. Soon I caught up with my recommended band of books i.e. more than 1000 words. I still went back to read books from lower levels, and the teachers were encouraging... They always said (in German of course), "Anything, as long as you read something!"