Worried about early 20s weight gain/“second puberty”

This is the first time I've ever heard the term "second puberty". Searching online it seems people do use that term. When you read things like this, you have to keep in mind that there's a lot of poor and inaccurate information available online. Like, if I google search "how to target belly fat" you'll find plenty of articles telling you how to do this, when it's impossible to spot reduce fat. The problem with online information is it's not peer reviewed and anyone can post anything. And when you "hear people say" things, they're often just regurgitating things they've read online or heard people say.
As for your question, I'm not an expert on women's hormones, but I'm thinking this isn't totally accurate. Women for sure will go through hormonal changes just before menopause but that won't be in your 20s. Sure, there will always be some people that have a form of medical issue, but for the vast majority, it will absolutely be lifestyle changes that cause weight gain in your 20s. Even if your hormones were to change, you still cannot gain body fat without a calorie surplus. You can't build a thing (fat) out of nothing. If you maintain your healthy diet and exercise, you have nothing to worry about.

I’ve read that it’s to do with your metabolism slowing down so you can end up eating in a calorie surplus despite eating and exercising exactly as you were before

I constantly see things online and hear people I know say that women go through a “second puberty” around the ages of 22-24 and gain a lot of weight and I’m nearly 21 now so I’m worried that uncontrollable weight gain is in my future. 😭
I’ve read mixed things online, some saying that it’s due to common lifestyle changes/“letting yourself go” so can be prevented by diet and exercise and other things saying it’s due to uncontrollable hormonal changes in the same way puberty is so there’s nothing you can do to prevent it.
My current weight is 6 stone 9 and I’m able to maintain that through just eating in moderation and doing a good amount of exercise but I’m low key terrified my weight will uncontrollably rise in the next few years. Is anyone able to confirm that there’s no such thing as a “second puberty” and it’s possible to maintain my weight from 20 for the rest of my life?

You are a very low weight for an adult woman. You should go to your GP for an assessment - at that weight you may be underweight which could have consequences for your future health and fertility.

You are a very low weight for an adult woman. You should go to your GP for an assessment - at that weight you may be underweight which could have consequences for your future health and fertility.

That's a good point. If you're anything over 5 foot tall you could actually do with gaining some weight.

I constantly see things online and hear people I know say that women go through a “second puberty” around the ages of 22-24 and gain a lot of weight and I’m nearly 21 now so I’m worried that uncontrollable weight gain is in my future. 😭
I’ve read mixed things online, some saying that it’s due to common lifestyle changes/“letting yourself go” so can be prevented by diet and exercise and other things saying it’s due to uncontrollable hormonal changes in the same way puberty is so there’s nothing you can do to prevent it.
My current weight is 6 stone 9 and I’m able to maintain that through just eating in moderation and doing a good amount of exercise but I’m low key terrified my weight will uncontrollably rise in the next few years. Is anyone able to confirm that there’s no such thing as a “second puberty” and it’s possible to maintain my weight from 20 for the rest of my life?