Goddamn, I love this show. If for no other reason than for dialogue such as, "You want it don't you, kind words?"…
Actually I think the casting of the mother is just right. If you look back at the scene where the angel first sees a picture of Kouki's mom his reaction is the same as yours. He was shocked that that she looked so young and beautiful to be Kouki's mother so I don't think she's playing her age exactly. Unlikely any ageism involved. She's supposed to look young for her age and it's also implied that she had hin when she was young.