Really?
I do know older people's metabolism slows but so do their eating habits, it shrinks.
My Mom is 60 and she's as trim as she was before. She never ever goes to the gym, she eats a good 3 meals a day and she's very healthy.

I think as you grow older your share of physical activity slows down as well.
Many times food habits might not slow in tandem with the lack of physical activity hence the imbalance in calorie intake and burning off.

I think ingraining a love for physical activity means not going to a stupid gym as it trains your mind to see physical activity as 'exercise' (=negative/chore/work) and bound to flop unless you're a really staunch, goal orientated person.
Instead, having a liking to be involved in pleasant activities and acquiring good habits which do not take mind bending or hypnosis for life.
My Mom loves going for walks. She goes for walks almost two hours a day to enjoy the weather and the scenary, morning and evening.
She has a cup of light tea in the mornings and walks nearby to have a traditional Asian breakfast of sweetened split green pea soup with dough fritters, reads the papers, then takes a good walk around the neighbourhood and parks before coming back home. Many times I go for these walks with her and get the same habits such as the love for long walks. We once walked and talked over 5-8 km (I think more but I'm not sure) in the evening till late and had yummy Chinese porridge for dinner before walking back home.
She has an excellent habit of never drinking soft drinks (hates the taste) and taking care of her skin through moisturising cream and ingestion of Chinese medicinal soups which supposedly improve health and skin.

As a result her health is excellent, she has a lack of wrinkles (you'd swear she's younger than 40) and has very smooth and fair skin, no skin blotches, wrinkles, nothing - maybe just very light smile wrinkles at the corner of her eyes, the rest of her face and arms/legs have very youthful skin.
Many times she has to show her ID for proof of age when using her senior citizen's card because they don't believe she is over 55.

It is possible that it is medical such as diabetes medication which made her grow fatter.
But generally its just lack of physical activity and eating habits.