Men over 30, what contributes to weight gain besides slower metabolism when you get older? Is it really hard to stay in shape?

Mid 50s here. One of the biggest factors for me has been how long it takes to recover from the wear and tear of being active. In my 20s I could be sore in the afternoon from exercise in the morning but be fine by evening. Now 48 hrs is typical. At my age I'm only focusing on maintenance and injury prevention.