Can I fly out of Thailand on January 16 if I arrived on December 18 with a visa exemption?

You can indeed fly out of Thailand on January 16 if you arrived on December 18 under the visa exemption. The 30-day count starts from your date of entry, so January 16 is your 30th day. It's crucial to ensure that your stamp confirms this duration when you entered, as mistakes can happen, and double-checking with the immigration officer upon entry is advisable to avoid any issues later.