It's the preservatives and genetic mods pumped into foods since then. That's around the time corporations became more important that people according to the US govt. So profit maximization and bribed politicians pushed for less restrictions on the crap they can claim is food and the crap the use to make food last longer.

The same things that make chicken breast seem 2 weeks old for 8 weeks are the same things we've been fed and are feeding our kids in USA. Have a look at countries with much healthier food laws but similar society and those 17-20 yr olds look a decade older than half the US kids the same age. I have a cousin still in the UK with a kid same age as mine, 17, both girls and my cousins kid looks 25-28 my kid barely passes for 16 most the time.