Bedroom issue

Okay so me and my fiancé have been together 4 years now and sometimes when we are having sex his penis doesn't always stay hard. Now I used too think it was me, until one day I finally asked him if something was wrong he said no it's because of his age now he's 35 years old I'm 23. Hess in the Army for about 8 went to Germeny and Irag but how is it that his age causes that to happen when my best friends husband it 37 and they never have that problem?