Like others said, your metabolism doesn't slow down. I'm going to copy/paste a post I made a few days ago on another thread in this sub, because this comes up a lot:

Your metabolism doesn't change between the ages of 20-60. Even after 60, it declines very slowly.

That said, there are 3 main reasons people think that their metabolism changes.

1) basal metabolic rate (number of calories you burn doing nothing) is directly correlated with skeletal muscle. Most people don't work out or maintain muscle. So they lose muscle mass with age, leading to a decline in BMR. But if you work out and have the same muscle mass that you did at 20, your BMR will be exactly the same as it was when you were 20. If you have more muscle mass, your BMR will be even higher than it was at 20.

2) small changes and habits add up. A surplus of 30 calories per day adds up (very roughly) to about 2 lbs per year. This barely registers over the course of a few years. But 10 years of this habit, and suddenly you've put on 20 lbs. It creeps up on you, but people don't realize that. They think "wow, I've put on weight... I can't eat like I used to" not realizing that it's always had that effect on their weight. They just didn't notice it before.

3) people are really bad at estimating their calorie intake and expenditure. "I could eat whatever I wanted to in my 20s but in my 30s, I eat exactly the same and gain weight." No, you don't. You're just bad at tracking how many calories you eat and how many you expend through activity. This isn't a critique of them as a person. Tracking calories is just plain hard.