septuagenarian, octogenarian - SEP-tua-jen-AIR-yan, OCK-toe-jen-AIR-yan - person who's in 70s, person who's in 80s
Example: "When Ms. Ertel turned 70, she became a septuagenarian; 10 years later, for a decade she was an octogenarian."
As a starter: and as a former teacher in English, I hope to add to reader's vocabulary by presenting a word a day in simple terms that anyone can understand - from current use in publications, radio,TV, etc. Remember: use a new word three times in a day and it's yours forever!