Your metabolism is a function of your caloric needs. Basically the amount you burn depends on the amount of physical exertion that you do added to the resting caloric consumption needed to maintain/rebuild your muscles. If your metabolism 'bottomed out' that means that you weren't burning many calories.
If you're unwilling to become very active, then it will be very tough to lose weight without drastically altering your eating habits.
These things change as you age. Without changing my diet or exercise at all it became more difficult to maintain weight in my mid-30s than it was in my mid-20s.
Unfortunately, since I already had a healthy diet and was active, the only way to do something was to become extremely active; something my body actually couldn't support due to various injuries through life. Meaning I gained weight.
The most reliable weight loss I've found so far is my pregnancy (down 20 pounds in 17 weeks). I suspect that won't be sustainable.
Yes, I have found that the people *most* likely to espouse this metabolism and calories in/ calories out have never been a female in their 30's or 40's or 50's.
The effects of hormones and insulin cannot be discounted. But unless you have experienced it, you won't understand it. (Especially the effects of stress.) And don't get me even started on menopause, which I haven't quite hit yet. No changes in lifestyle, BOOM 10 to 15 pounds. Hello hormones! I've also read some interesting books that discuss the difference between being "formerly overweight" and "never overweight" and how your body processes food differently. (For one thing, you have more fat cells. But also, you've permanently damaged your body. For this reason, many people who have been overweight cannot eat as many carbohydrates as people who were never overweight. But I digress.)
And the honest truth about time and exercise are often ignored.
I have a friend in her 70's who exercises 2 hours a day and eats 1200-1300 calories a day. She gained about 15 pounds after 70 that she could not take off. Her doctor says "you are 70!" (This is a woman who gained 10 lbs during her one pregnancy and had an 8.5 pound baby. Always trim and fit, but had to give up tap dancing and tennis in her 60's due to injury.)
She has taken off 7 pounds by adding a "high intensity" 45 minute aerobic interval workout 3x a week, on top of her 2 hours a day. She said "I've pretty much accepted that I will have to do this forever to keep the weight off".
Imagine, 2 hours a day PLUS 3x of 45 minutes of interval workout. Don't know about you, but *I* don't have that much time.
My old boss is 58 and was a distance runner and cyclist. With injury and major illness, he cannot run because he cannot feel his feet anymore. So he has gained some weight (well, the not running and the medication). He still bikes 100 miles at a time, but his body is used to that.
(On the flip side, a different old boss, same age, is 6' tall and 138 pounds and cannot gain weight. He's also a cyclist. On a bad note, his bones easily break.)
I found that I was able to maintain a decent weight in my late 30's when I was running, but then I got injured. First the hip, then the ankle, then the knee. I cannot run distance anymore. In fact, I can't run at all. Well, what's wrong with that really? Lots. For one thing, I was running 6 miles in an hour, but can only walk 3 miles in an hour. That burns approximately half the calories. I don't exactly have 2 hours a day to work out. I do have time to do some - walking, swimming, weight training, yoga. But there simply aren't enough hours in the day to increase my workout time.
My caloric intake is around 1200 to 1400 calories a day. It's very hard to get the kind of nutrients your body needs on less than that. In fact, 1200 calories is the *minimum* recommended.
The "it's just math" people tend to be overwhelmingly male and young, in my experience. And are the folks who can drop 10 pounds by going on the "six pack diet" (you know, cutting back from a six pack a day to one beer).