Wah, all the explanations so clean... I am dissappointed...
Normally when ppl say this nowadays, they are jokingly referring to a woman's sexual libido as she ages.
At 30, a woman has the libido like a hungry wolf.
At 40, her libido is now like a starving tiger.
At 50, her libido is like a ravanous black hole. Also, she has probably mastered the technique of "kiap kiap" until cannot pull out...