The definition of Central Ontario seems to have changed over time.
Historically, central basically meant between eastern and western Ontario (Toronto wasn't the sprawling metropolis it is today). This was reflected in the postal codes. "L" covered a central region except Metropolitan Toronto - including Halton, Peel, Dufferin, Simcoe, York, Ontario, Durham and Victoria counties.
One residue of the old county system is that Cobourg has K postal codes and Port Hope has L postal codes.
Today Central Ontario seems to mean basically cottage country - Simcoe, Muskoka, the Kawarthas, Haliburton etc.
It leaves modern-day Northumberland difficult to define. It seems more oriented toward Peterborough/Kawarthas than the Quinte region, yet it borders Lake Ontario.
The old distinctions are still used by Canada Post:
https://www.canadapost.ca/tools/pg/m...Gaddress-e.pdf