My reaction to your question is that if a person is overweight and eating a bad diet, the benefits of dieting and exercise far outweigh how hard it is to be overweight. You will improve the kind of nutrients you get and your body will benefit, you'll protect yourself from illness or even early death, and you'll feel better and look better. I think most damage to the body is repairable, or at least is far outweighed by improved health. I'm hesitant to say the following because it can be used as a way to put off dieting and exercise, but I've also read that fat cells can be reduced in size by diet and exercise but don't go totally away. (Only things like liposuction take them out for good. ) That said, lots of people lose weight and keep it off despite having put on some added fat cells when they were overweight.
You'll never know. Too many factors play into this. Some people start out their lives overweight. Some get overweight young. Some not until they get older. Causes for it differ. Medication can cause it. Diet can cause it. Heredity plays a part. Food allergies and intolerances can play a role. So this isn't really a question to ponder, as you can't really answer it. What you can answer is that if you stay this way you will definitely increase the chances of a painful older life. Or no older life. All the rolls of the dice on this one will come out, change the lifestyle.
If you already have damage from being overweight, such as heart disease or diabetes, chances are those won't be reversible, but if you don't already have any health problems, losing weight can certainly help prevent them.
I've read that we're never too old to get healthy and although I don't necessarily agree with it, you're young enough to do yourself a world of good by losing weight and doing everything you can to keep it off.
I'd recommend that you get a thorough check up, including thyroid panel (since thyroid controls metabolism), blood glucose and insulin levels, etc to make sure you're starting off with a (relatively) clean slate.
Good luck and I hope you'll stick around and let us know if there's anything we can do to help you.