According to Osho, the phrase 'dirty old man' names a pathology created by sexual repression: the body has grown old and wants rest, but the mind—stuffed with unfulfilled, repressed desires—still lusts. This mismatch breeds a leering, ugly energy. Lived naturally, sex wanes by midlife and transforms into innocent love and compassion—the truly 'clean' elder.
It means an old person seems creepy because his body is tired but his mind still craves sex from years of repression; if we live sex naturally, old age becomes gentle love.