No one knows the exact cause of rosacea. Genetics play a big role. Fair skinned people with Celtic ancestry are prime candidates. Bill Clinton and Prince Harry are good examples.
We do know the outer layer or skin barrier is compromised and super sensitive.A demodex mite has been discussed for some years. Higher numbers of these mites are found on Rosacea skins but whether they are a cause is in question.
The most recent research which is continuing today has found elevated levels of anti-microbial peptides Cathelicidin and Stratum corneum tryptic enzymes in Rosacea skin. These findings indicate there is an immune system weakness.
To read up on these studies Google Dr Richard Gallo. Many references are available to check out.
Sun damage and a diet rich in acidic food and drink can accelerate the symptoms and perpetuate the condition. Perhaps this is why more young people are showing signs of rosacea. Today's diets tend to be very acidic to the body.