Responses
I wrote this on my tablet so there is Grammer problems
Its probably because of Mana. I think the Royal family has magic. In other stories, people with mana tends to live longer because of magical powers. As shown before, the princess' mother and grandmother can use magic. So maybe magic also exist in the human world. When the queen visited the demon king castle, she travelled from the human world to the demon world. Maybe the ending of this story is Suya becoming queen and connecting the demon world and the human world to end the war.
Like if u remember during the arc where they went to the spring and went to see the Mc grandmother, they said how her great grandmother was still alive AND STILL looks slightly young for her age. SOOO my guess is that since her family can live extremely long lives; the age slow as a result which cause her to look young despite being a grown ass woman