No one made it up explicitly, it just follows the formula for the rest of the decade age words like octagenarian

You got a problem with it? What are you, some kind of trenagenarian?

The Romans did or at least they created the base word structures. Primus, secundus, tertius, quartus, et cetera could all be compounded with the suffix genarian to create an age bracket specific word. For example I am a secundagenarian.

Completely off topic, but had to look up what a teenager is, and what a useless word. Who decided we needed such a long word to say "in their teens"