That is exactly the reason why but unfortunately wives will not admit it to their husbands.
It's that simple, women are more hungry in their 30s. That's when guys start to decrease their libido. The sex gets old and its just becomes a 30 second session for the man.
There seems to be two peaks for women.
1- Teenager to early 20's.
2- They become calmer after this, then the surge appears in 30's.
Actually in late 30's to early 40's when they fear of losing female-hood.
But this is not a rule. There are always exceptions to the rule or usual.
There are women in 50's and 60's who are really 'Khuwaar'- the word someone mentioned above.
And about men, Most men do not lose their interest until very very late. In fact when they get to 40 and above, in order to try to compensate for their fear of losing it based on lack of knowledge, and hence they try to hit any woman who comes across their way and get excited. (useless and unnecessary overcompensation since men do not have menopause!)
There is a saying: Man and Horse never get old.
Again exception to this does occur.
Addendum:
Before I hit the save button you had some discussion about 30 seconds with PCG.
Let me comment on it.
Both women and men when excited enough can get climax very quickly.
In fact if a woman is very very excited, she may even get to the end of story before the man get to do anything. Remember someone telling us the reality of wet dream? I agree with that.
Now men have a bad scenario here:
1- Men cannot fake or hide orgasm.
2- Men are considered losers, inconsiderate, in-experienced, fool, less macho if they get to achieve satisfaction quicker than woman.
But it is not true. Even though women may be disappointed at times, they still feel happy to have their partners happy.
(Awaiting for PCG to throw shoe at me and ducking! :-) But it is still true what I said)