For the non Garfield die-hard, this strip is a wind-up for our favorite orange devil's birthday on-or-about June 19. Around this time we will be treated to many references to how very old Garfield is becoming. The implication in this strip that perhaps Garfield's body is deteriorating with age and the turnaround joke that Garfield was always in poor shape forgets key Garfield history: he was a lot fatter and more disgusting in 1978.
The funnier part of the strip though, is Jon's behavior. It's about when someone tries to make a wise-ass remark, then realizes that it didn't work, and maybe the truth is even worse/funnier. The mode of delivery is funnier than the message, and check out Jon's acting in panel two. That serene/sick expression as Jon accepts that the joke he made about his friend becoming crippled with age is true, that's what Garfield birthdays are all about!