In your seventies
Definition 1 of 1
Pro Tip 1/3Age Precision
The term is less precise than stating an exact age, and implies a range. 
He's in his seventies, so he's seen many changes in the world.
Pro Tip 2/3Potential Underestimate
When someone says 'in their seventies', it could mean 79, not just early 70s. 
Though in her seventies, she could outwalk many younger folks.
Pro Tip 3/3Collocation Clarification
Adding 'early', 'mid', or 'late' gives a clearer picture of the specific age. 
My uncle, in his mid-seventies, still runs marathons.