Both characters use the 老 ('old') radical. 耄 traditionally meant about 70-90; 耋 meant about 70-80. Combined, 耄耋 is the literary cover-term for 'extreme old age,' typically eighties and nineties. The pairing appears as far back as Han dynasty texts.
memory
A common pun: 耄耋 sounds nearly like 猫蝶 ('cat + butterfly'). Traditional paintings of cats and butterflies are auspicious gifts to elders precisely because they pun on 耄耋, wishing the recipient long life.
