I don't think their parents, three of whom lived well into their 90s, ate or avoided anything very special. They kept Kosher, ate plenty though not excessively and enjoyed life.
Never eating anything special in 21st Century and early 20th Century are night and day. It's not a valid comparsion.
The food environment has changed unrecognizably
* less home cooking
* prepared ingredients even when people do cook
* way cheaper food
* an almost uncountable variation of foods high in sugar, fat and low in protein and fibre i.e. junk food.
If you want to bypass a mammal's ability to control bodyfat introduce them to junk food; it'll bypass satiety signaling even when full. Try getting lab rats fat on rat chow and then dry with milk chocolate, ice cream and fried chips, see which happens faster and which are healthy after.
The variety of foods will also encourage over eating.
An obese teenager today in all probability had a disadvantage to a 1900 teenager in that his metabolic dysfunction probably started in the womb if his mother had a crap diet. I remember interviewing midwifes about 12 years ago; when asked the hardest part of their job "the women are just getting bigger and bigger" was typical answer. They loved seeing the Polish girls walk in.
In truth most people who have struggled all their lives with diet always will. 10% loss of body mass appears to be the ceiling for most.
The only chance young generations have is to eat like their great grandparents did, before the damage is done. Given the proliferation of junk food, marketing, misinformation that is an uphill battle.
Focusing on macro, fat good carbs bad and vice versa isn't really helpful; you can build a healthy diet with pretty massive variations in macros for a healthy person. Just eat whole foods with plants dominating plate and your as good as done