My husband never wants to have sex with me anymore...

Ok, so my husband acts completely uninterested in me sexually anymore. I know without a doubt that he loves me and there isn't anyone else. I honestly can't help but think it's our age difference. I'm 25 and he's 31. I know it's not much, but maybe enough to make a difference? We've been together for 4 years... I've tried talking to him, and he tries, but I can tell it takes an effort and that kinda makes me feel like crap. The only time he actually seems engaged is after a few drinks. If we are sober I have to come onto him and he lasts like 2 mins (no exageration), and then I'm left even more frustrated than before... I'm lucky if I get something out of him 1 time every other week, twice a month isn't enough for me. Twice a week isnt enough for me!! I don't know what to do....