Boston
To answer your question, there is no such thing as too old. I have clients over 60 that are spryer and in better shape then men in their 40's. They are also intriguing, interesting and distinguished gentlemen who have lived life to a point where they are comfortable with themselves. I adore and respect their wisdom and knowledge, which in turn allows them to relax and be wonderful lovers.
While older men may want to see a younger lady for her youthfulness, there is seldom depth to the encounter. I know of a few older gentlemen who have been ‘taken for a ride’ because they become so enthralled with being able to pay a younger woman that they lose sight that it’s just a fantasy for hire.
The younger/older session is typically more of a transaction than a satisfying experience. There is no way for an older gentleman to appreciate the close mind and body experience in a 1/2 hr to 1 hr appointment. Perhaps an extended session could help with this, but the typical scenario leaves many older men feeling rather empty. Marketing is simply playing with words to get the human mind to respond. I 'market' to a select group of gentlemen who not only assist me financially, but genuinely round out my social life and the lifestyle I have chosen.
As we mature the sexual experience goes way beyond the visual and physical experience. It begins with seduction of the mind and ends with toes that curl. The look, the touch, the feel-slow and intoxicating foreplay that leads to mind blowing orgasms. Younger women often lack the knowledge and skills to deliver such service. It is difficult for a woman to adapt her style from one appointment to the next.
Sorry to appear rude, but I've been around a few years catering EXCLUSIVELY to gentlemen 40+. There is no way to effectively switch from seeing younger men to older men if a woman sees several gentlemen a week. I have held steady with my standards over the years, and that is why my appointments are two hours at a one hour rate. The second hour is gratis because it is often necessary to ensure a fully satisfied and pleasurable experience for the more mature gentleman.
Those of us (and there are a few) who see only older gentlemen are able to pace our pulse to match that of our partner. Like my opinion or not, I truly believe it is an art form. I guess it's a form of geriatric tantra.
Here is an interesting article. Now I know why I love well educated mature gentlemen. 
Kate
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http://www.velvetoasis.com