When you're younger, your metabolism is different. It's much easier to remain slim whilst eating all manner of rubbishy foods.
But this changes as you get older. And, it's usually the case that we become more sedentary. We don't get 'playtime' where we run around for 20 minutes twice a day every day!
However, on top of all this is the fact that our diets have changed significantly over the last 30 or so years.
Anyone who knows me will realise that I am now going to bore for Britain about low carb 
But. Think about it. Think about your local high street. How many of the shop units there comprise food outlets? How often these days do we expect to have a take away/eat out/buy snacks during the day?
30 years or so ago it would all have been very different. There might have been a fish and chip shop, and the odd cafe (very much a 'greasy spoon' type of place). But apart from that, there would have been very few places where you would have gone to buy takeaway food. Food would have been consumed almost entirely at home, or at the workplace, in the works cafeteria. We would have eaten 3 'square' meals a day, and wouldn't be snacking during meals.
Anyone who is the same age as me (I'm now 54) would also remember that drinking fizzy drinks was a real rarity - we used to wait for the Corona lorry to come round - and buying one, one litre bottle of orangeade was a real treat that maybe happened once a week, if we were lucky.
There also wasn't a diet food industry. Low fat foods just didn't exist.
And then, in the 1980s, it was decided that fat was the problem, and we were all advised to eat low fat foods. Low fat foods became a massive growth area for manufacturers. And guess what? Obesity levels have soared ever since.
So. We are now in a position where we are all trying very hard to eat low fat foods. But low fat foods use a lot of sugar (i.e. carbs) to make them palatable.
We also eat out and eat on the go much more often. Most of what you will eat these days (any coffee shop, cafe, food concession) will be heavy on the carbs. If you're eating lots of carbs this will also mean that you end up being hungry a lot during the day, which means you will want to snack at around 11 and 3/4pm, so you'll be eating even more carbs. But don't worry, because what you're eating (mostly, isn't it?) is low fat.
But it's all still high in carbs. And, worse, if you're not trying to eat low fat, it will be high in fat and high in carbs. Which is a complete recipe for disaster. And it's a vicious circle. Eat carbs - get hungry - eat more carbs - get hungry - eat carbs ...
Carbs are the problem. Cut out the sugar. Reduce the amount of sugar, potatoes, pasta and rice you're eating, and go back to traditional meals of meat and two veg. Stop drinking fizzy drinks and eating chocolate/sugary snacks. Take more exercise if you can. All of this will also mean that you're not as hungry, so you will be eating less.
And also accept/recognise that with age it does get more difficult, because your metabolism slows down.
I am now 54. I embraced low carbing properly (having flirted with it for a while) a couple of years ago, and lost two stones. I now wear size 8-10 clothes, and have maintained this weight for a year. (Christmas bad behaviour not withstanding 
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If you want to know more/join in, the next low carb Bootcamp starts on 13 January.