The best diet advice I can give relates strictly to my own experience:
I.E. - Male, over 65 - H 5'9" - Current W - 160 lbs.
If you're in that category, more or less, but weigh 175 - 180 and would like to lose weight, say 15-20 lbs to be slimmer - e.g. size 36-38 belt- then give this a try:
Eat pretty much whatever you like, but keep daily calories as low as 1,200/day, preferably 1,000 or less; protein around 50 or so - less if you can, and limit carbs to under 25-30 gms/day - you should easily lose 2-3 lbs. per week - no sweat.
I've accomplished it several times in recent years, and can keep my weight down to 160 easily, by religiously maintaining these params.