timebandit,
It’s all about the metabolism, it slows down with age.
If you continue eating and drinking what you used to eat and drink when you were in your 20's, the extra kilos will be easily gained.
Realistically and no matter what "they" tell you. You only have two options:
- increase your metabolism rate
- reduce the food and beverage intake

The first one, you achieve with regular cardio exercise;
The second one should never be the solution itself.
A combination of both is the only option you have to continue living healthy and in shape.

all diets are an illusion, even if you loose weight, your metabolism rate sharply decreases and as soon you start eating normal portions, you'll get as fat if not more than before.