There might be something to the theory of a woman's sexual peak occurring during her 30's. Of course the variables and physiology are going to be different for everybody.
I have been married for 10 years now and my wife is turning 31 this year. We have three children. Two of which were....accidental and one which was an actual attempt. All of which are completely wanted.
With our second child we tried and tried and tried and tried and it became a tedious task. Sucked the joy right out of sex for both of us.
Sometime last year my wife experienced her first orgasm and our sexual encounters have become much much more exciting.
So from personal experience I pretty much completely agree with Quesi on this.