Twenties start at 20, Thirties start at 30 etc. But i see the downvotes and have no reason to argue. If teens start at thirteen fine by me. Could also be a cultural thing, never heard anyone say what you're saying but makes sense.

Yeah i think cultural/language difference is the root cause. Either way it's not really important or that relevant. In Dutch the word for a teen literally translates to 'tenner'. But, like English, the first 12 words of the numbers are base 12, and only from thirteen does the 10 come back in the name. You generally go to high school at 11 or 12, so the age 13 has no significant meaning here.

Slightly related, in politics of other countries i do see that age more often, for example Ursula von der Leyen just announced she wants 13 to be the legal age to use social media. For me and i think many people here that seems like random age, but in this light it does make a lot of sense; it is the first age that has teen in it.