Comment by OJFord

A not dissimilar age thing I've found interesting is from India: it is (or can be? Big country) typical to age yourself by the year of life you're in, i.e. 1-based rather than 0-based, basically.

On his birthday my grandfather in law said 'I am x years old! [...] Oh, no, x-1 complete.'

It's funny these kinds of arbitrary systems for things we have, and never question, until you suddenly stumble into one that's slightly different.