Actually, there is a time-frame element to this issue. By the time Martin Van Buren died, people were using this term synonymously with "aged [x]," so that's where the mistake lies. My research in New England and New York State shows that the majority, at least, of instances of "AE" and "in [their] [x] year," they were used literally. This faded out during the 18th century and by the 19th, it appears most people didn't know the difference. There are many instances of published gravestone transcriptions that ignore the actual wording and say "aged [x]," which, for Colonial New England, at least, is problematic. Best to find the original where possible, of course.