There was [an interesting thread](https://www.reddit.com/r/AskHistorians/comments/ur8qy7/what_was_the_deal_with_the_important_of_cleaning/) on this in /r/Askhistorians recently. Emphasis on washing behind the ears of infants goes back at least to the 18th century. So certainly long before glasses were common and at a younger age.
Why cleaning behind the ears became so prominent culturally, I’m not sure. I’d guess part of the reason would be: 1) it’s an area that can trap dirt; 2) in the past children would have their head washed less frequently than now; 3) it’s an area that’s not visible to the child; 4) it could be visible to other people.
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