We don't actually know when Michael's real birthday is, do we, since we found out that he and Lindsay aren't twins. Lindsay said in Development Arrested that her birthday was in a month, but that was before she found out she was adopted. Sitwell confirms that that's her real birthday, even though she got the year wrong. So when was Michael born? And how did the Bluths claim them as "twins" when their birthdays are months apart?
The show is not consistent with the kids' ages, either. In a scene from the extended Pilot (on the DVDs), GM says he's 13. Later in season 1, George Sr. says that GM is 14, This would suggest that he was born in 1989, not 1990. Plus Tracey was already pregnant during the 1989 wedding, which would tend to support 1989 as his birth year. His age is not mentioned in season 2, but in season 3, he has a birthday and states a couple of times that he's 16. But this is contradicted by the marriage certificate and Michael saying his son is 15 in the finale. So GM's age keeps fluctuating, and now in season 4, they make him 22, so he's born in 1990 again. It wouldn't bother me that much, except that GM said once that he was older than Maeby.
There's less data about Maeby. Her birthday is briefly mentioned in season 1, only in the context of Nana sending checks on her birthday every year. We also hear about her being "made in a cup like soup" but get no dates. In "Afternoon Delight" Maeby says she's 15, and GM throws her a 16th birthday party in season 3. Yet season 4 seems to suddenly age her up; she tells Kitty that she's 17 not long after the Queen Mary. She says that she's 23 by the time she seduced Perfecto in 2012, which means she was born in 1989, contradicting the marriage certificate. I'm kind of shrugging it off as a meta comment about how much she lied about her age when she started working at Tantamount Studios. Maybe she's lost track of the truth, and we know she's not good at math.