1. Because as they have more Knowledge of the World and their Minds are better stor’d with Observations, their Conversation is more improving and more lastingly agreable.

2. Because when Women cease to be handsome, they study to be good. To maintain their Influence over Men, they supply the Diminution of Beauty by an Augmentation of Utility. They learn to do a 1000 Services small and great, and are the most tender and useful of all Friends when you are sick. Thus they continue amiable. And hence 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 thro’ more Experience, they are more prudent and discreet in conducting an Intrigue to prevent Suspicion. The Commerce with them is therefore safer with regard to your Reputation. And with regard to theirs, if the Affair should happen to be known, considerate People might be rather inclin’d to excuse an old Woman who would kindly take care of a young Man, form his Manners by her good Counsels, and prevent his ruining his Health and Fortune among mercenary Prostitutes.

5. [OP]

6. Because the Sin is less. The debauching a Virgin may be her Ruin, and make her for Life unhappy.

7. Because the Compunction is less. The having made a young Girl miserable may give you frequent bitter Reflections; none of which can attend the making an old Woman happy.

8. [thly and Lastly] They are so grateful!!

https://en.wikipedia.org/wiki/Advice_to_a_Friend_on_Choosing_a_Mistress

(http://www.autoadmit.com/thread.php?thread_id=5719256&forum_id=2#48899325)