But it doesn't have to be that difficult. The airline contracts with the hotels. It can choose to contract with the many (really most) that will accept guests at age 18. It can also, due to its purchasing power, insist as a condition of the contract, that a hotel that usually requires age 21 accept its vouchers for guests age 18 and up. It's the same deal as the corporate rental car contracts that allow drivers younger than 25 without any additional charges.

If the airline chooses to use only hotels that require guests to be 21, then it probably should prioritize passengers <21. And yes, that might be awkward on the Thanksgiving weekend. But it would be the airline's choice that caused that.