As you get older, it gets harder and harder to burn off calories, metabolisms slow due to hormonal changes (men declining testosterone, women hormonal changes through menopause), and weight gain is common. This can be especially true like @op, who said they were naturally thin, likely while they ate well and exercised, they had a natural body weight that tended towards being thin. I have read plenty of stories of fashion models, whose diets consisted of things like junk food, potato chips and booze, who were really thin without trying, never exercised, and then when they got older found out how hard it was to keep the weight down (not saying all models are like that, of course, just using one example of what happens).
I agree with others, if you otherwise haven’t changed your lifestyle, you may want to get your thyroid checked, just to make sure, it doesn’t hurt to see if that is involved.
A couple of other things:
1)What kind of exercise are you doing? For example, if your exercise when you were younger was walking a lot, that may not be enough now, you might have to change your exercise routine…to keep the weight off, you might have to do more intensive cardio and /or weight training, long, slow cardio like walking may not be enough if that is what you always did.
2)I would recommend journaling what you are eating, and look most importantly both at quantity and kind. As you get older, you may find that you can less tolerate carbs, so if you always ate a lot of bread or rice or the like, it may be hurting you, even whole grains can be a problem. Or if you eat a lot of fruit,t hat can cause weight gain, because of the sugar. There is eating well, but there is also eating well based on your body and what it is like now…and also be careful of trying to lose weight by not eating enough, being hungry, you can put your body in famine mode. Try changing what you eat and drink, if you drink diet soda, stop drinking it, if you regularly drink alcohol, cut back and see what happens. If you eat a lot of grains and rice, try eating less of that and more green vegetables and lean protein, and see what happens.
It sucks, being on the other side of 50 the body just doesn’t respond like it used to. The other thing is, don’t use the scale much, don’t weigh yourself ever day, do it once a week at most. You can gain weight if you weight train, so things like losing body fat can actually cause you to gain weight. I share with you the battle, in the last year, thanks to work being over the top and some health issues, I stopped regularly exercising and also with eating became a little less disciplned (I eat well 95% of the time, but given I am not exercising, maybe too much or too many calories), and put on close to 30 pounds. If anything, know you aren’t alone:)