Thank you for the context. It looks like the article got cut off at the bottom of the page, sadly.

A study in BMJ Oncology last year reported a sharp global rise in cancers in people under 50, with the highest rates in North America, Australia and Western Europe.
Doctors are racing to figure out what is making them sick, and how to identify young people who are at high risk. They suspect that changes in the way we live—less physical activity, more ultra-processed foods, new toxins—have raised the risk for younger generations.
...
The risk of developing some cancers at a young age has increased for each generation born since the 1950s, studies suggest. One found that people born in the 1990s are at double the risk for early-onset colon cancer and four times the risk for rectal cancer, compared with people born around 1950.

These excerpts may suggest some kind of lifestyle or environmental (as in immediate surroundings) cause among first world countries. Finding a singular factor to explain everything seems unlikely.