In your fifties
Definition 1 of 1
Pro Tip 1/3Age Range
Specifies a 10-year span someone is in, not a specific age. 
My mom is in her fifties, so she is somewhere between 50 and 59.
Pro Tip 2/3Life Stage
Often implies a stage in life with grown children, career stability, or approaching retirement. 
At this get-together, most participants are in their fifties, discussing retirement plans.
Pro Tip 3/3Health Implications
Sometimes associated with a certain health and fitness level or age-related changes. 
He's in his fifties but runs marathons, defying the age's stereotypes.