I think you can improve concentration.
Limit screen time(endless screens and flickering images can't help),egg timer activities,learning an instrument they love,enjoying a hobby they love,Lego etc.Must be loads more.
I have twins 8.1 always happy to sit for hours looking at books and puzzling things out,always very able.His twin never sat still for 2 mins in the womb let alone out.He was a nightmare re concentration but I think he's overtaking his twin to some degree in the concentration stakes.
Several people have told me learning an instrument really helps with concentration and it has massively with dtwin1.He adores the piano and has really started concentrating more in everything since starting.He's loves fishing which he's doing more of and Lego. He's quite self disciplined now.
I just think it takes effort and encouragement to improve concentration but it can be done.