Just month and day alone: Nakajima Saki: February 5, 1994 Toda Rinne: February 6, 1981 Lehua Sandbo: February 7, 1983 Kago Ai: February 7, 1988 Yajima Maimi: February 7, 1992 Hagiwara Mai: February 7, 1996
If you take account the year, the one closest in age with me was Iwashima Manami (SI☆NA).
And among those who are still around... Tanaka Reina
Morito Chisaki is currently the closest to my birthday. If you take years into account, then it would be Ayaya (I've always found it amusing that I was born the same year as two amazing singers and am as good a singer as Sayu was during her audition ).