But this is only if you assume at the start of the anime each idol is aligned to their age that you would find on their profiles since the start of 2nd Vision. The anime never gives us a definite answer to how old each idol really is and all I can do at best is speculate but when I took the time to lay out the possibilities the results were actually quite interesting. So here's one way to look at it.
If you assume that each idol is at the age specified on their official profiles as of January 1st of the most recent year, then each idol would be the following age:
Haruka - 17 (April 3rd)
Chihaya - 16 (February 25th)
Miki - 15 (November 23rd)
Yayoi - 14 (March 25th)
Iori - 15 (May 5th)
Yukiho - 17 (December 24th)
Makoto - 17 (August 29th)
Ami/Mami - 14 (May 22nd)
Azusa - 21 (July 19th)
Ritsuko - 19 (June 23rd)
Hibiki - 16 (October 10th)
Takane - 18 (January 21st)
If we follow this as the standard for the anime then a couple interesting things happen. A lot of the idols become older than you would think and we can actually then map out the timing of some of the events of the anime using the idols' birthdays.
Judging by the sakura blooming in episode 1 I would assume the episode takes place sometime in spring between March and early May. At this point some of the idols would then already have had their birthdays. Chihaya would already be 17 and Takane would already be 19. Haruka, Yayoi, Iori and the twins all have their birthdays in the spring also but without an official date we can't be exactly sure how old each idol is in episode 1.
Episode 2 doesn't really give many clues as to what time of the year it takes place in but if I had to guess, at least a few weeks have passed since episode 1 with Producer being recently hired so it would likely be some time in May. Episode 3 however takes place during a summer festival so it's probably June by then.
Episode 5 gives a definite clue though to the timeline. During episode 5's beach episode Ritsuko states to Azusa that she's still a minor when offered alcohol. Since Ritsuko's 20th birthday is on the 23rd of June and the age of majority in Japan is 20, that would mean episode 5 would have to take place some time in June before the 23rd. The irony being Ritsuko would become of legal age only a short time after.
The only time anyone is given a confirmed age is in episode 12 when Kotori says to Producer that Miki, although an idol is still just a 15 year old girl. Meaning that sometime after their first live and before the end of the anime Miki does in fact turn 16. Episodes 13-21 would then fall under sometime around autumn to the beginning of winter.
Following this then we're led back to the Christmas party and Yukiho's birthday which would have to be her 18th. Unfortunately the candles on the cake probably don't amount to her actual age.
Episode 25 of the anime is on New Year's and the final scene after Producer is released from the hospital fully recovered is a few months after the New Year's live. About an exact year has passed since episode 1 with 765Pro going to see the sakura in spring. At the end of the anime then the idols would have now become the following age (oldest to youngest):
Haruka 18 or 19
Iori 16 (possibly 17)
Yayoi 15 or 16
So by this standard a lot of the idols would be older than you might actually think during the anime. By the end of the anime Takane is actually legally an adult even. Chihaya being 18 years old at the same time as Haruka, Yukiho, and Makoto is quite surprising as well but I can realistically see her that way from her character.
But for all I know the idols could also be their profile ages at the start of the anime or some crazy other. Haruka would then be younger than both Yukiho and Makoto, and Iori would be younger than Miki. The only thing I can be sure of is that Miki is 15 by the start and later turns 16 before the end. The characters of im@s were designed 10 years ago and the birthdays were likely selected, perhaps not arbitrarily but at least without the thought that there may be inconsistencies if they were ever put in an anime. Although it can only be speculation at best, having an idea of how old the idols really are is a fun topic to think about and I wouldn't be doing my job as their producer if I didn't know my idols' actual ages and how long I have to wait until the twins are of legal age.