Sex every 3 months

I'm 45 my husband is 47 (no kids) we have sex every 3 or 4 months to me this is normal because allways been like this since we got married 15 years ago we are happy this way we live each other but my close friends are telling me that this is not normal at all I never worried about it before I will like to know what other women think or if some body live the same way as we are .