The airport of La Paz (Bolivia) is very high. It is about on 4200 meters (13780 feet). It is perhaps one of the highest with international traffic. I flew from Buenos Aires to La Paz on a 737-200 in the 1980ies on Aero Lloyd Boliviana.
I don't know if there are higher elevated airports in the Himalaya countries where jets can land.
The highest airport in the world with civilian flights is the one in Leh, in Indian Kashmir. It is actually an Indian Air Force Base but it has a large civil enclave. There are around 10 flights in all daily using B737's and A320's to this Tourist Destination.