I can't say "Seu filho tem dezoito anos"?
OK, I will report it then! Duo marked me wrong and it was the very last question and my last heart :(
That sucks. So unfortunate nobody uses "seu/sua" the correct way anymore.
I guess people just prefer to use 'dele/dela'. Also it is funny how in both Portuguese and Spanish 'seu/sua' is used for second person, but in Portuguese it is informal whereas in Spanish it is formal
French word 'Vous' can mean singular or plural. German word 'Sie' can mean she, they and formal you. So yeah, many languages have similar phenomenons