The 24 month period mentioned is not a prescriptive regulation but is a "safe harbor" given by the IRS in Rev Proc 2008-16. 

The NFPA and the ICC developed and publish codes through a prescriptive process, mostly because some got hurt or destruction and devastation occurred.

However safe harbors are not prescriptive and there are others who think a year is OK. 

You are allowed to ask for proof of prescription.

There is a safe harbor (sort of) at 2 years with specific use requirements from the IRS but again the safe harbor is not meant to be prescriptive but only descriptive of one period that would be acceptable.Best advice - say your explanation to the mirror. 

This would be called a prescriptive easement.

Hi Ian,In almost all states there are limits to what an unlicensed people can and cannot do, just think of the medical profession, can you prescribe prescriptions?

Plus raising kids when you have no benefits I know for about the first 10 years or so all we could do was major medical so doctor visits  prescriptions etc were not covered. 

Your listing language is fine, and you aren't doing any of the stupid things many landlords do, like use very prescriptive language or listing lots of rules. 

While you cannot ask WHY they need the animal, you can request proof of prescription.