One thing that leapt to my attention is the difference between your squat and DL. Its a pretty big gap. My DL is around the same as yours (500 conventional, also sumo on a really good day) but my squat is 425 (sleeves, walk-out) and 460 (wraps, monolift). The gap between bench and military is big-ish too (I've got about the same max on military, maybe a little more, but my bench is a good 10 to 15 lbs lower than yours).
Without having seen your lifts I'd hazard a guess that somewhere there's an imbalance and that's what is holding you back; or somehow you've adopted styles for each lift that really don't suit you and just stuck with it (which seems unlikely since it'd feel like crap).
Given that I'm 30, and you say you feel fine and have the other key stuff dialled in I really doubt its an age thing.
I keep coming back to that squat/DL and BP/military difference. It occurs to me maybe your midsection isn't strong enough all over? I mean, it isn't going to be weak given your DL but perhaps you aren't getting tight enough? I know for me in squat and press tightness is absolutely essential whereas with DP and DL I can get away with a tad less (especially for sumo).
Like I said, not having seen you lift I can't do much other than guess but my top two guesses would be an imbalance or lack of tightness.
Hope that helps.