When did Adams and Jefferson last see each other in person?
I don't think they saw each other in person again after 1801 -- following Jefferson's election as President but before his inauguration because Adams left town before Jefferson was sworn in. Their friendship started to fall apart when Washington was President and really started to fray during the 1796 election, when Adams defeated Jefferson, which meant Jefferson ended up as Vice President under Adams because the system for electing the President and Vice President was ridiculous prior to the enactment of the 12th Amendment. The 1800 election was even nastier than 1796, and Adams retired to Massachusetts without sticking around for Jefferson's inauguration and they completely fell out of contact for over a decade.
Fellow Founding Father Benjamin Rush tried to get Adams and Jefferson to reconcile once Jefferson left office in 1809, but was unsuccessful until one of Jefferson's neighbors told him that he had visited Adams and Adams had said, "I always loved Jefferson and still love him," which led Jefferson to let Rush know that any beef that he had towards Adams was definitely squashed and that he'd respond if Adams wrote to him. Adams sent Jefferson a letter on January 1, 1812, Jefferson responded on January 21, and they restarted their remarkable correspondence and kept writing each other until they both died on the very same day, July 4, 1826, which also happened to be the 50th anniversary of the ratification of the Declaration of Independence, which will always be one of the craziest coincidences in the history of human existence.