Oh yeah, some info on the girls in age order (I don't know a lot, but oh well):
Mai HagiwaraBD: February 7th 1996
Info: A Minimoni movie appearance, some backdancing and a musical with Megumi Murakami (I'm not sure if it had other H!P kids, too). Mai seems to be mostly known for sharing a name with a JAV star. Heh.
Chisato OkaiBD: June 21st 1994
Info: Chisato appeared in Koinu dan no monogatari movie. Besides that she's done some backdancing in performances. And... Stuff.
Airi SuzukiBD: April 12th 1994
Info: Airi was in a one-time unit called Aa! and sang in a H!P All Stars c/w song. She was in Minimoni the movie, as well. She seems to be the most well known non-Berryz H!P kid.
Saki NakajimaBD: February 5th 1994
Info: Saki was the girl who called Rika her idol in Utaban's H!P All Stars episode. She's done some background dancing and briefly appeared in Koinu dan no monogatari movie.
Megumi MurakamiBD: June 6th 1992
Info:
She was in ZYX, appeared in Maki's Yokohama shinkirou PV, sang in a c/w to the H!P All Stars single and will be in the Sexy otona jan shuffle group this summer. She's also been in a musical, but I don't know details about that. She also had a small role in Koinu dan no monogatari.
Maimi YajimaBD: February 7th 1992
Info: Maimi's the girl who always does well at H!P Sports Festival. She was in ZYX and had a small role in Koinu dan no monogatari movie.
Erika UmedaBD: May 24th 1991
Info: Like Megumi and Maimi, Erika was in ZYX and appeared in Koinu dan no monogatari.
The ages of the girls:
Mai Hagiwara - 9
Chisato Okai - 10 (turns 11 later this month)
Airi Suzuki - 11
Saki Nakajima - 11
Megumi Murakami - 13
Maimi Yajima - 13
Erika Umeda - 14
And the height order :