I'm 60 years of age and have been the same weight for nigh on 40 years, more or less. Thankfully I have no health problems.
I think the trick is to be constant in what you do to keep your shape. Regular exercise is key, as is adjusting that exercise to fit the need.
I walk EVERY day without fail, 1 mile minimum and at weekends go longer walks, anything from 4 to 15 miles. I play tennis, table tennis and do twice weekly stretches, runs and exercises.
I never eat more than I have to which means that I cook the right amounts usually and order smaller dishes in a restaurant. Each day I have a salad - cucumber, tomatoes, lettuce, spinach, carrots - a combination - and only eat meat say twice or three times a week.
If you're going to diet do it sensibly. Get advice, yes, from those qualified but do not go for fad diets. In my opinion first thing is to:
* look at what you're eating and drinking now,
* take note of the amounts and times you eat and drink
* think about what it is you enjoy eating which is obviously fatty and unhealthy.
Cut down gradually. Adjust exercise accordingly. No crash diets or foolish one food diets!!
Stay safe. Seek health advice if you're going to really get into keeping in good shape. Be careful. And good luck.