Power dynamics are present in all relationships. To a large degree, power is based on what you want and what you are able to achieve. This principle applies to a wide range of issues including sex. The more sex you want, but the less able you are to get it, the less sexual power you have.
A study of sexual behavior done in 2010 suggests that a sexual power shift occurs between men and women when they reach age 25.
Women have an easier time than men having sex before age 25. After age 25 the power balance seems to shift. Women older than 25 have new competition – younger women.
Men face exactly the opposite problem. Younger men tend to have a more difficult time having sex before age 25. Their problem? Young men are competing with more successful men.
As much as we like to deny it, men are attracted to younger women and women are attracted to successful men. Women hold more power when they are youthful while men hold more power when they are successful.
A graph of this power shift can be found here.