The answers to previous questions indicate it's possible to apply for and complete a PhD even at age 65+ (example, another example).
If these applicants did undergraduate studies at a conventional age of 20+, by age 65+, their undergraduate supervisors are probably retired or deceased. Who writes the recommendation letters then (especially if the applicant's job did not involve research)?
If the answer to this question is also applicable to all students whose supervisors are retired or deceased (e.g. a campus shooting kills all of a student's supervisors), I'll edit the question to include the broader scope.