The Effects of Ageing
Males reach their peak sexual capacity during their teenage years and it is common for sexual responsiveness to diminish somewhat with age, though the vast majority of older men are still able to perform sexually and there is no evidence that enjoyment or sexual fulfilment declines. It is known that testosterone levels decline from their peak in the early 20's throughout life, and this is thought to be the main reason for the changes observed. The basic physiology of the cycle remains almost the same, though there are some commonly observed changes that occur during each phase, as outlined below.
The Excitation Phase
Because of certain effects of ageing the man may take longer to establish an erection or be unable to achieve this at all. This phase also tends to continue for longer in older men as they need more time to reach climax.
Heart rate, blood pressure and the intensity of breathing is not as pronounced and the erection may not be as complete or firm, but this does not seem to hinder the feelings of sexual pleasure in any way.
The Plataeu Phase
As a man ages he has greater control over the timing of his orgasms, as a result he spends much longer in this phase. He is able to maintain an erection over a much longer period of time and the urge to ejaculate diminishes somewhat so that it is less urgent to enter the orgasm phase. Many males require more direct genital stimulation to remain erect, as the effectiveness of psychic stimuli falls.
The Orgasm Phase
In a younger man it may take some minutes for the erection to wane, whereas now it disappears almost immediatetly. Since the reponses are not as pronounced the ejaculation can be much weaker and less intense. Ejaculatory fluid volume also decreases with age.
The Resolution Phase
Again since the physiological responces are not as prominent in the previous phases it takes much less time for the body to return to its normal resting state. Conversely, the refractory period increases and may be unable to respond to stimulation for a great amount of time.
Libido
A decline in libido and increase in age is directly correlated, but the cause may have many different roots. Often, issues with sexual function is more a direct consequence of the emotional or psychological state of the individual. Worries about health, finance quality of lifestyle and matital conflicts may all lead to a decrease in sexual desire. Although the loss of libido may also be related to a decrease in testosterone levels which may be increase by a supplement of endogenous testosterone.

References
The information on the page is sourced from:
- Blazer DG, Steffens, DC. (2009) The american psychiatric publishing textbook of Geriatric psychiatry, 4th edition. American Psychiatric Publishing.
- https://www.askdrkhaled.com/normal%20male%20sexual%20function/6The%20Male%20Response.htm
- All images are sourced from flickr under the creative common license agreement