Not that old. They start with 747-400's that are just about to need D-checks and would be in need of interior refurbishment. I think one of the China Airlines 747-400's being used struck it's tail during a landing. The fact that the cargo section will not be pressurized makes it an ideal candidate to be converted.
It is the oldest of the classic 747-100/200/300 aircraft that are best suited for conversion to beer cans. The 747-400 has a two-man digital cockpit which is being heavily upgraded for the 747-400LCF's. The 747 being a long ranged aircraft, has fewer cycles than shorter ranged aircraft like 737's with equivalent numbers of hours. It's much cheaper for Boeing to acquire used 747's than to build new ones for conversion to the LCF.