Why is pedophilia considered a mental disorder but gerontophilia is not?

Gerontophilia : While gerontophilia isn't formally classified as a specific mental disorder in dsm-iv, it is clearly a paraphilia and potential criminal behavior. There might be various underlying causes that need to be looked at when diagnosing this problem, but why it hasn't been included as a formal mental disorder may be more a result of its low prevalence rather than the actual reality of it as a disorder.