Between 1950 and 1959 the word ELDER appeared 37 times.
Between 1960 and 1969 the word ELDER appeared 24 times.
Between 1970 and 1979 the word ELDER appeared 446 times.
Remember that during the 1950s and 1960s they didn't use the term "elder" and considered it a clergy title like "Reverend" or "Father." It only makes sense that they started using it a lot more when they decided it was OK to apply the term to their own people.