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.
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.)