This is irrelevant without knowing when most people incur their disabilities. If most people over 75 developed their disabilities before age 65, for example, then this makes the phenomenon more surprising, rather than explain it.

This states no difference between the government and employers’ insurance plans that would explain why older people are less likely to receive disability benefits. It doesn’t say that either payer is less generous and more likely to serve older people.

This would explain more people living past 75, but not why fewer of them receive disability benefits. It doesn’t say those medical advances have made older people less likely to suffer a disability.

This doesn’t offer an alternative explanation. The author does not say a person’s predisability income impacts their chances of receiving disability payments.

This offers an alternative explanation. Older people are less likely to receive payments because they are more likely to have lived past the retirement age.