The results of the new research found that men aged 45 to 54 spend on average 7.8 hours per weekday sitting down, compared with 7.4 hours for the over-75s.
Time spent sitting at work is the main reason for their sedentary time.
The team also found that only the youngest group of men — 16 to 24-year-olds — are significantly less sedentary than the over-75s on weekdays.
Most of the time spent sedentary in this age group is spent in front of a TV or screen.
At the weekend, those aged 25 to 54 were the least sedentary, sitting for between 5.2 and 5.7 hours a day, and in contrast the over 75s were the most sedentary, at 7.3 to 7.4 hours a day.