I'm over 40

The good (?) news is that metabolism is pretty constant from our 20s into our 60s:

https://pubmed.ncbi.nlm.nih.gov/34385400/

That's metabolism in the strict sense, i.e., the chemical processes that occur within a living organism in order to maintain life.

The implication is that the changes that take place as we age, that can lead to increases in our bodyweight, are not unavoidable things, but rather caused mainly by factors entirely within our individual control.

I don't know about you, but as I went from my 20s to my 40s and beyond (to 66 now), my basic lifestyle tended to get less active. In my college years, I walked everywhere for transportation (or rode my bike), played active games with my friends or danced and such for fun, had a physical job. Even in my early professional years post-college, my jobs tended to involve more movement, but gradually involved more sitting over the decades. Early on, I was doing home improvement projects more, working on getting yard/garden in good shape, cooking at home more vs. eating out, and doing some home maintenance chores that I now hire out. For me, all of that stuff added up.

In addition, I was doing more social eating alongside the less-active daily life stuff, and the stuff 88olds mentions - that portion creep, the creep toward richer foods. It all adds up, and the weight creeps on.

Until my 40s, I also wasn't very active post-college exercise-wise. Not only does that mean burning fewer calories in itself, but muscle mass tends to decline with age if we don't work hard to keep those muscles. Muscle only burns a few extra calories (per pound per day) than fat mass, but as we lose muscle it becomes less easy/fun to move, and we avoid it more, accelerating the process of losing strength/muscle and reducing calorie expenditure through movement.

I think that this effect - not challenging our muscles - is an even bigger risk for women than men, though technically it can occur for men, too. But women are a little more likely to be stereotyped into jobs with lower physical demands, not to be expected socially to do the 'heavy work" around the house, and stuff like that.

If one has a history of yo-yo dieting, that can accelerate the down-spiral of muscle loss and activity reduction even more.

Like I said, the good news is that all of this is reversible, because it's stuff we control. (That doesn't mean changes will be quick - they do take persistence and patience, not to mention conscious effort.)

We can challenge our muscles, increase strength (surprisingly quickly) and muscle mass (much more gradually). Doing that makes it easier and more fun to move. One of the best, most efficient ways to boost strength and muscularity is through strength training. You can learn more about options for that from this thread:

http://community.myfitnesspal.com/en/discussion/10332083/which-lifting-program-is-the-best-for-you/p1

Despite the title, that thread does include programs (like bodyweight exercises) that don't require a gym or lots of equipment. And don't worry about "getting bulky" or overmuscular, or having to lift some giant weights in order to progress. Women who are very muscular have worked consciously to do that over many years (and sometimes used dangerous drugs along the way). Most sleek/fit women these days do at least some strength exercise. And all we need is enough weight/resistance to manageably challenge our own current strength level. If that means starting with smaller weights, or bodyweight, that's completely fine.

Another thing one can do, besides the obvious exercise (strength and cardio) is think about ways to increase daily life movement. There's a whole thread with ideas for that here:

http://community.myfitnesspal.com/en/discussion/10610953/neat-improvement-strategies-to-improve-weight-loss/p1

Since even fidgeting can burn a couple of hundred extra calories per day (not that I'm necessarily telling you to fidget!), it can be surprising how much we can accomplish just by changing up our daily life routine in some manageable ways that don't take a lot of extra time.

Speeding up your metabolism isn't a probable thing to achieve, in a short time scale. Of course, getting good overall nutrition may be a help to increasing daily energy level, but there isn't any magic food regimen or anything like that.

But speeding up metabolism also isn't necessary. There are achievable things a person with commitment can do, to burn more calories, and - of course - manage the calories eaten, in order to get weight creep under control.

Personally, I was overweight to obese for around 30 years, but lost weight using MFP at age 59-60, and have been at a healthy weight since . . . able to eat around 25% more calories doing so than MFP estimates for a woman my age (66). I'm not a magical unicorn. You can make changes in your habits, reach your goals, too.

Wishing you excellent results!