When does your metabolism typically begin to slow down and gaining weight becomes easier?
Normally when there is a hormone change, injury or lifestyle change. When I was around 45, old injuries developed arthritis, and I could no longer do some of the things I enjoyed doing. Bike riding over 5 miles, extremely long hikes with back packs, just became too painful. I noticed I had to watch what I eat a lot more carefully.
Most people report a drastic change around 25-27 years of age.
Ever notice that's also the same time those people get a good job and spend their days sitting at a desk? I don't know of any studies on this (maybe something is out there, but nothing I've chanced upon), but I think hypatia summed it up nicely. At least anectdotally, when you take a good look at the people commented about having a harder time losing weight, etc., as they approach 30, you'll also note that they've had a lot of lifestyle changes as well. My biggest weight gain was right after I stopped being a student in that age range, but it had little to do with my age, and a lot to do with 1) being paid enough to afford to splurge more often on junk food, and 2) no longer having to walk all over campus for classes, and instead being more stationary working at a lab bench or desk. I especially gained weight once I moved to a cheaper apartment too far to walk to campus (hence the reason it was cheaper in an expensive university town).
Separate names with a comma.