Truth be told a person's metabolism slows down gradually with age... a slowdown does not occur when you reach a specific age.
So much more than age effects one's metabolisim, general health, diet, medications, activity, etc. You can employ many strategies to fight this uphill battle - eat a healthy balanced diet, increase activity, and focus on maintaining the health of your vital organs (heart, liver, kidneys).
If you have experienced any sudden changes that are causing concern - such as suddent weight gain (or loss), changes in appetite, or changes in your digestive process, its likely not "age" causing the problem and warrants further attention.
I think it's a gradual process that starts with the end of puberty and dogs us for the next 6 decades or so...slow and steady, like a Chinese Water Torture!
There are some theories that people's metobolism doesn't slow down, but rather thier lifestyles just slow down gradually over time. I personally don't know what to think but I have assumed it does slow down over time but I think it's a lot more gradual than most people think and it really is a combination of lifestyle and genetics combined rather than just age.
I have just turned 60 and work out harder than I have at anytime during my life. At age 25, I could eat (and drink) 3500 calories a day and hold steady at 180-190#, If I went steadily TODAY at 3000 calories I'd gradually creep up to 300#. Holding at 212# now takes about 2300 - 2500 calories. My musculature is less now than it was but not by MUCH.
Age is a cruel mistress, but of course the alternative to old age is not AT ALL appealing.
(My heart rate is a good 10-15 BPM slower than when I was a kid.)
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