There isn't a double standard, the name for male cougars (or Mougars) is simply not as popular.

A cougar is urban slang for a woman who dresses a certain way, acts a certain way and is sexually interested in younger men.

Popular media such as Saturday Night Live has made the term popular with comedy sketches such as these:
http://www.hulu.com/watch/57940/saturday-night-live-cougar-den

-quote-
They're called 'Cougars', and we're not talking about wild animals. ... These vivacious women don't think age should limit who women date. ... Turns out, many young men are attracted to older women. ...
-endquote-

Why are women who date younger men called cougars? Starting at the beginning, the Tupi of Brazil called the Puma concolor by a word meaning "resembling a deer" in their language, the world resembling "cuguacuarana". This passed to Portuguese and thence to French "couguar" and finally the English cougar. The name was used for the large cat in question for a few hundred years until about 1999 when it appeared in western Canada as a name for women who pursued younger (generally by 8 years or more) men. It showed up as the name of a dating web site for that purpose, it is not clear which came first, the slang or the website name. In 2001 it was used in the title of a book about this sort of cougar. This caused the word to spread widely but it became even more popular after regular use on a popular television show.

In general, words come into use when people need them to express themselves conveniently. The cougar phenominum is new enough that people want a word to refer to it. If older women seeking younger men becomes as common as older men seeking younger women the word will probably fall into disuse.

There is a name for men who date younger women. Lucky!

According to Wikipedia, "Cougar" is a slang term:

-- quote --
Cougar—a woman over 409 who sexually pursues younger men, typically more than eight years her junior.10 The term appears to have been coined by Canadian website Cougardate.com11 and has been used in TV series, advertising and film. The 2007 film Cougar Club was dedicated to the subject and in Spring 2009 TV Land aired a reality show called The Cougar. A new show called Cougar Town, explores the difficulty and stigma of many so-called "Cougars".
-- /quote --

Hope this helps!