The type and amount of erotic stimulation necessary to summon an erection varies from man to man and also with age.
Adolescent and young adult males usually have no problems obtaining erections, which may occur with (or without) minimal sexual stimulation. A healthy 18 year old, for example, can get an erection simply by fantasy or other non-contact sexual stimuli. Young men can have two or three full sexual encounters, from erection to orgasm, in a short time with little or no foreplay required. Although progressive changes in a man's sexual response usually occur by age 30, he can still easily develop an erection during kissing or foreplay. However, after age 50, he will usually need more direct sexual stimulation to achieve a firm erection.
This gradual slowing in sexual response as the man becomes more mature and sentimental has its rewards. More often than not, the love play and the sexual relationship become more meaningful and more pleasant. In older age, a healthy man is usually able to maintain good erections but tends to require more time during foreplay to achieve one. Note, though, that spontaneous erections occur during sleep at any age. For any man, regardless of age, to have an erection, he generally needs an anxiety-free, relaxed atmosphere - this means no performance demands.