A year younger. The 1st arcade game (which was based on the OT cartoon) considers Leo 16 years old, and the other Turtles 15 years old, but the cartoon
itself doesn't give anything concrete.
Not only Zach, but Buffy Shellhammer who was 15 years old in "Poor Little Rich Girl" acts more like bratty child than a bratty
teenage girl, and maybe even Vernon's nephew in "Too Hot To Handle" (if he's supposed to be a teenager). It's weird but the show depicted most human teenagers as children (not only in maturity, but in height as well, if we are assuming the Turtles are around 5 feet tall [152 cm] and haven't experienced their growth spurt yet
). One could argue about how the fighting and life-threatening circumstances made the Turtles more mature than their peers, which is logically plausible (not only in the OT but in other TMNT universes), but for the OT at least, I don't think the show's narrative was intentionally designed to make that implication considering the targeted audience and tone.
For me, I assumed the reason for making these human teenagers more like children, was to appeal more to the kid demographic and exaggerate their idiosyncrasies for the show's comedic tone.