Marital intimacy does not diminish with age. It really depends on the couple however I think there is ever-lasting love out there. I am sure there are couples that are as intimate today as they were when they first got married. I hope that couples can be married for a long time and still be intimate.
Intimacy is not just about sex, so if it is a healthy marriage, the intimacy or closeness or sharing between two people should generally grow. However, the body is affected by aging and the expressions of passion and closeness may change or even diminish while the love and intimacy does not.