noun
one's 70s; seventies
Refers to the age range from 70 to 79 years old, or to the decade of a person's life in their seventies.
祖父は70代ですが、毎日散歩しています。
My grandfather is in his 70s, but he goes for a walk every day.
70代の人口が増えている。
The population of people in their 70s is increasing.
70歳 refers specifically to the exact age of 70, while 70代 covers the entire decade from 70 to 79.
Compound of 七十 (ななじゅう, 'seventy') and 代 (だい, 'generation' or 'era'), used to indicate an age range.