Sensitivity of the G-spot and Epicenter depends on the DHEA and testosterone level in the bloodstream. My first girl friend was very sensitive there when I met her at her 19. According to the first Chinese medical book, the Yellow Emperor's Classics of Internal Medicine (Huang-Ti Nei Ching), written 5000 years ago, women reach their sexual peak at age of 21, but the adrenal function increases sharply at age of 14. However, the female libido is influenced by both testosterone and the ratio of testosterone-to-estrogen (TE). At 21, both etsrogen and testosterone levels in the female body are highest in the female life, but this does not mean that the female libido reaches the peak point. The female libido starts to increase when the estrogen level begins to ebb but the testosterone level reminds constant. This usually happens to women from 28-42 years of age. So long as the female body retains a high level of testosterone, her G-spot should be very responsive. The female testosterone and DHEA are produced in the adrenal cortexes. The female testosterone level varies with the DHEA level very closely. Although estrogen is produced in the same place, but its production is triggered by a different hormone massager released from the Hypothalamus/Anterio Pituitary in the brain control center. In fact, this is how my herbal supplements works, triggering the right hormone messager for sex.
On the sexual potential scale for men and women, the function of the adrenal cortex determines the sexual potential for the female of all age and for the male after midlife. The male sexual potential is determined by the adrenal and testicular functions.
The sexual potential for a 19-year old female will be around 7 out of 10; for a 20+ female, 8-9 out of 10; for a 30+ female, 9-10 out of 10 if the adrenal cortexes function properly; for a 40+ female, 7-9 out of 10.....
The sexual potential for men is reflected by the ejaculation frequency without causing sexual exhaustion. An optimal ejaculation frequncy should promote the hormone production system and improve the male health and body strength.
For middle-aged men, the situation is very complicated. It depends on how they replenish their bodies. If they don't get replenishment, the male sexual potential will be 3 out of 10, allowing to ejaculate 2 or 3 times in one month. A robust man of 50+ can still ejaculate every 4-5 days, 6 out of 10.
Personally I, 48, can still hold a power-up (fully ballooned) penis for 60-90 minutes in the morning intercourse and ejaculate twice a week with a thick semen without feeling wearness and exhaustion. I have fully experienced the power of my herbal supplements. Who taught the power of herbs? My Mom advised me to take a tonic herbal win every day after my turning to 40. My mon makes herbal win for my father who is 72 now. He still works in the farm, out-performing any young men. It seems he still has a lot of hormones in his bloodstream to carry out any heavy-duty works. However, I have sucessfully searched my own way without alcohol!
Hence, it is very difficult to evaluate the sexual potential of a man after midlife, but the erection hardness and ejculation frequency will tell you how much he still has. Here I should emphasize the erection hardness over the ejaculation frequency, because only the hard erection allows a man to give his woman multiple orgasms in one love session and to continuously stroke her longer than 40 minutes without a leak. Thus, he can have sex more often without ejaculating to give her more orgasms than what she demands.
The 5000-year Chinese sexual bible called SU-NUI-CHIN suggests the following ejaculation frequency - the sexual age scale - for men:
30 years old, once every 1-2 days;
40 years old, once every 3-4 days;
50 years old, once every 5-10 days;
60 years old, once every 10-20 days;
70 Years old, once every 30 days or never again.
When I compare myself with the sexual bible suggestion, I, based upon the "sexual age" scale, am 40, but not 50, years old. Basically, I have rolled back my sexual age about 10 years younger. However, I am looked like a 30+ year young man. I attribute my younger body to the power of herbs and my mom's advice. Mom always knows the best!

PS (updated 12/11/1998): The other secrets for a man to become younger are:
1. having sex everyday, but regulating his ejaculation frequency (don't ejaculate if possible), so that he can be energized by intercouse without feeling exhausted and weary after each ejaculation - the stimulation of the brain-adrenal-testicular axis of the endocrine system.
2. making his woman achieve multiple orgasms as possible as he can, which requires the penile ballooning (power-up) effect, so that he can extract her sexual energy (bioelectricity) from her sexual power center - Epicenter, to power his penis and to replenish his Automatic Nervous System, and his penile cells and neurons can get more nutrients, oxygen and hormones - DHEA and Testosterone at the same time.
I have practice these Taoism teachings since I got married in 1976, leading to my creation of Sexual ChiKong Interourse in 1977.