Teenage and younger seiyuus are nothing new. Yuzuyu in Aishiteruze Baby was played by nine-year-old Tsuzurahara Miyu. And Hirano Aya was 14 when she played Lumière in Kiddy Grade.
This season we have two 16-year-olds giving fine performances, and an eleven-year-old giving a convincing performance as a six-year-old.
Seven-year-old Murasaki (not nine), the heart of Kurenai, is played brilliantly by 16-year-old Yuuki Aoi 悠木碧. Yuuki-san already has quite a resumé, including three films and a number of TV dramas.
Playing Murasaki, her voice is true to the character and very expressive. It occasionally goes off the deep end, but so do most seven-year-olds. Yuuki-san has already had some anime roles, under her previous name of Yabusaki Aoi. She was Marika, Yuzuyu's rival, in Aishiteruze, and has a continuing role as Yumeno Koto in Onegai My Melody.
Also in Kurenai, six-year-old Chizuru is the victim of Murasaki's enthusiastic game-playing in episode five. As Anime Princess wrote, she sounds like a real six-year-old. And she is played by a real eleven-year-old, Imagawa Reina 今川怜奈. Imagawa-san has already been in TV dramas and commercials, but this is her first anime.
It's appropriate that these two young seiyuu should be acting opposite Sawashiro Miyuki, who started her own anime career at fourteen, as Petit Charat in Di Gi Charat. Will these two become pro seiyuus of the same quality as Miyokichi? Hard to say for someone as young as Imagawa Reina, but Yuuki Aoi's acting talent is obvious, and I expect to hear more from her.
The third young seiyuu I want to mention is at another level already. Hayami Saori 早見沙織 broke into anime at 15, playing the heroine Momoka in Touka Gettan. Now 16, she is playing Kou, a powerful but clumsy young priestess, in Wagaya no Oinari-sama. Some posters on the Japanese forum 2channel have said that they thought they were hearing Mamiko Noto, and were surprised to find out it was this young newcomer.
Hayamin has a voice that is already fairly professional, and gives subtlety to the Nagato Yuki-like expressionlessness of Kou. Her singing voice is more than slightly professional. When she left the booth after recording the ED for Touka Gettan, the staff spontaneously applauded. Her ED for Wagaya no Oinari-sama is my favorite anime music of the season.
The 2channel consensus is that as a seiyuu she is good and getting better, and as a singer she is already at a high professional level -- the kind of level I hear when I listen to Sakamoto Maaya's OP for Escaflowne, recorded when she made her anime debut at 16.