Wiktionary
midteens
n. The period around the middle of the teenage years, such as ages 15 and 16
Usage examples of "midteens".
The boy was in his midteens, long-legged and driven by panic, darting into the hedgerows in hope of finding a hiding place.
He put the boy in his midteens, and knew, too, that fifteen years before, Jilseponie had indeed been pregnant.
Her hair had been blond once, but that had changed in her midteens, by the look of her.