I'm thinking that your boyfriend may bothered or depressed about something. You did mention that he says that he's "...tired and stressed." And, that could explain a lack of sexual interest... but, only in the short term because people usually recover from fatigue. Unless, maybe... he's simply not be feeling well. Other than the sex, have you noticed other changes in the relationship? How is his overall mood? I doubt that the age difference is factoring into this. I suspect that whatever is going on with him is between the ears... and not anywhere else.
trobpah, what were his past relationships like? Has he ever been in a long term relationship with a woman, that you know of?
The age difference might be something to consider - he's 32, and not married - ever been married?
He may just not be interested in sex, in general. Or sex with women specifically, I don't know.
A healthy, heterosexual man who's 32 who has a hot babe girlfriend who is 23 should be more interested in sex than he is -
You might need to jar him out of the status quo by being a little more aggressive. Obviously, I don't know either of you well enough to know if this would work in your situation. Quite often, men want to be "attacked", that is, with good timing on your part, you would surprise him with an unhurried hand job or a b*l*o*w*j*o*b as he sits in a chair reading, for instance, as you sit on the floor between his legs. Now, I realize that this is not solving your question entirely, but it is meant to jar him, outside the bedroom, causing him hopefully to think more sexually about you.