Our body and mind goes through a lot of changes as we age, these are both physical and psychological and affect every aspect of our 
life. When we are young, we tend to think that our body and looks are going to last forever and old age seems far away but age always catches up with us, sometimes without us even noticing it.
One area where you will tend to notice a significant difference due ageing is sexuality. Men and women will both see changes happening in their bodies. For men erectile dysfunction (ED) or impotence will become more common, he may loose the ability to get and maintain an erection during sexual intercourse or find that his erection is not as hard as it used to be when he was younger. He may also have less stamina to perform for longer in bed.
Women on the other hand may feel loss of libido (loss of sexual desire), dryness in the vagina ( which may cause pain during intercourse) etc.
The can be many reasons for sexual problems in older people, some of them are physical, such as Diabetes, arthritis, chronic pain, heart ailments etc. Other factors like specific medications and alcohol abuse may also contribute to sexual problems.
Then there are some psychological issues that things like the loss of stamina and strength (which is a natural part of ageing) in men and the perceived loss of beauty and looks in women, can create. These can make some people feel uncomfortable about sex as there is a loss of self esteem and confidence and this directly affects their sexuality in an adverse way.
What is important to realize is that these changes are normal and are easily treatable in most cases through counseling, medication and exercising. You should always consult your physician or a health care professional like a sexologist if you feel that you are having sexual problems. Talk to them and provide your complete medical history to them so that they can guide you and suggest the right type of treatment to help you overcome these problems.
Remember that just because you are older doesn’t mean that you cannot enjoy a satifying sexual life. Just make the appropriate lifestyle changes and seek professional help to rekindle the passion in your relationship.
You can read more about sexuality and ageing here.