From Wikipedia:
HyoJung (Choi Hyo Jung; Korean: 효정

- July 28, 1994 (age 21)
JinE (Shin Hye Jin; Korean: 진이

- January 22, 1995 (age 20)
Mimi (Kim Mi Hyun; Korean: 미미

- May 1, 1995 (age 20)
YooA (Yoo Yeon Joo/Yoo Si A; Korean: 유아

- September 17, 1995 (age 20)
SeungHee (Hyun Seung Hee; Korean: 승희

- January 25, 1996 (age 19)
JiHo (Kim Ji Ho; Korean: 지호

- April 4, 1997 (age 18)
Binnie (Bae Yu Bin; Korean: 비니

- September 9, 1997 (age 18)
Arin (Choi Ye Won; Korean: 아린

- June 18, 1999 (age 16)
So yeah, only one was even underage.