I saw this flying through O’Hare on our way back from Costa Rica. First time for everything.
Each state in the United States has laws setting their minimum drinking age, which years ago varied between 18 to 21 years old. The federal government indirectly imposed a minimum drinking ago of 21 by withholding federal road funds to states with lower minimum drinking ages, coercing states to pass laws setting 21 as their minimum. The previous patchwork of laws required airlines to serve alcohol only to those 21 years old and older to avoid breaking laws of states flown over.
It may be that the same applies to cannabis, and that it’s more than just you destination, until cannabis is nationally available. Supposedly it’s illegal to take cannabis on a flight, though international drug tracking has much larger consequences.