I’d blame lack of exercise more than smoking, processed foods, vitamin D, etc.
When we were kids we had 3 channels of tv and board games, if there was nothing good on TV we were outside playing games and riding our bikes, there’s now hundreds of channels of tv, endless streaming, YouTube, thousands of video games, combine that with parents worry of what will happen if kids are outside and you get a bunch of kids sitting on the couch rather than running around.
Makes me glad I was a kid when I was.