That question requires some interpretation.
My age limit to be in a woman. As old as I can get and still produce something to do the job.
The age limit for someone to put it in? Hmmm. 20 years ago, with very few exceptions, the idea of finding a woman over 45 desirable would have been remarkable. But I'm going to be 45 myself this year and perhaps it's due to my generation having enjoyed such good nutrition and medical care or maybe it's just that standards slip as the rest of me does but there are some remarkably hot 45+ year old women out there now.
Also, from this Straight Dope column about Benjamin Franklin:
" Was Franklin a misogynist? He wrote a long-suppressed but now famous essay on matrimony that, taken literally, would give you reason to think so. Franklin argues that matrimony is the natural state of man, but for those preferring to cat around instead he lists eight reasons that "in all your Amours you should prefer old Women to young ones," to wit:
"1. Because they have more Knowledge of the World . . .
"2. Because when Women cease to be handsome they study to be good . . . there is hardly such a Thing to be found as an Old Woman who is not a good Woman.
"3. Because there is no Hazard of Children, which irregularly produc'd may be attended with much Inconvenience.
"4. Because through more Experience they are more prudent and discreet in conducting an Intrigue to prevent Suspicion.
"5. Because in every Animal that walks upright the Deficiency of the Fluids that fill the Muscles appears first in the highest Part. The Face first grows lank and wrinkled; then the Neck; then the Breast and Arms; the lower Parts continuing to the Last as plump as ever; so that covering all above with a Basket [!], and regarding only what is below the Girdle, it is impossible of two Women to tell an old one from a young one. And as in the Dark all Cats are grey, the Pleasure of Corporal Enjoyment with an old Woman is at least equal, and frequently superior; every Knack being by Practice, capable of Improvement.
"6. Because the Sin is less. The debauching a Virgin may be her Ruin . . .
"7. Because the Compunction is less. The having made a young Girl miserable may give you frequent bitter Reflection; none of which can attend the making an old Woman happy.
"8th and Lastly. They are so grateful!!"
In my opinion the above was written with tongue in cheek, but you be the judge."
http://www.straightdope.com/columns/read/2522/whats-the-true-story-on-benjamin-franklin