Methyl oxide

From Memory Alpha, the free Star Trek reference.

Methyl oxide is a gaseous compound that is breathed by Lorillians until they are four years old, at which time they are weaned onto oxygen. (ENT: "Broken Bow")

There is no chemical compound named methyl oxide in real life; perhaps the writers meant methylene oxide (common name formaldehyde, H₂CO), or perhaps it was simply a made-up name.