It could be partly genetic, but it's possible that it's at least to some extend caused by too much skin being laid over too little bone. If his face were to widen a little, perhaps those folds would become less prominent. Just a guess.

Muscle use and balance. Pull your cheeks back and then lips toward your nose as you open your mouth, and these get more pronounced. I notice some people talk with this expression and it only deepens the folds since the folds form when the face does this. Mine have gotten less pronounced through proper oral posture and good lip seal.

What helps is to stretch your lip and cheek skin forward by puckering before lip suction hold, and then letting the lip suction come back and apply gentle pressure to the teeth. 

Lol I have the same. I remember someone mocking me back to when I was like 11 yo about that. I also have diagonal lines under the eyes, a bit as bags under the eyes but they're not the same thing. To me it is clearly just a sign of "soft tissue normal growth versus hard tissue bad growth", as my bones did not develop well whereas the skin just continue its path.

I'm very young and it does not seem to be a problem with opposite sex (that's kinda what it is about, right ?), and it did not got better with the overall upshape I had with mewing.

@EddieMoney , I don't really understand what you mean by "this exression [when talking]", can you develop ? I'm definitely conscious that I have bad muscle use when talking, as my jaw retrudes, and double chin forms - when it should protrudes. Most probably have to do with pterygoids atrophy, as they protrude the mandible, and mine has been in a retruded position for years.

I'm upping this topic, the fact it could be muscle related aroused my curiosity

Anyone did facial exercises with success ?

@bogdar

I haven't been doing any facial exercises but over the course of improving my tongue suction hold, lip seal, posture, etc., I have observed the muscles of my face becoming more prominent. The effect is subtly visible throughout the entire face, but the growth of my masseters is the most apparent.