I'm confused with the ages.

Well on the 老友记 wikia, it says that Rachel was born in May 1970, which would make her 24-25 in the first season. Monica was born in April 1969 which would make her 25-26 in the first season.
In season 7, Rachel would have been 30-31, so I think the writers must have made a mistake because I think that means she should have turned 31. Mind you, the writers have made plenty of mistakes before - Ross, Chandler and Emma's birthdays were always mucked up.