GA LTT OVERSHOT ALT CROSSING RESTRICTION ON APCH.

Narrative

WE WERE BEING VECTORED FOR AN APCH AT DTW AND WERE TOLD WE WERE #8 FOR THE APCH; EXPECT 21R ILS. I SET UP 21R ILS FOR THE FLYING PLT AND WE CONTINUED BEING VECTORED. AFTER ABOUT 10 MIN; THEY CHANGED OUR RWY TO 21L. I GOT OUT THE APCH PLATE AND SET UP 21L FOR THE PLT. 21R LOC HAS ILS DME BUT 21L LOC DOES NOT; SO I HELD THE DME FROM THE RIGHT SIDE SO WE WOULD KNOW OUR DISTANCE FROM THE ARPT. WE INTERCEPTED THE 21L LOC AS INSTRUCTED AND FLEW IT INBND. APCH SAID; 'MAINTAIN 5000 UNTIL ROBBI CLRED FOR ILS 21L. CALL THE TWR NOW.' I CALLED THE TWR AS INSTRUCTED AND THE PLT FLYING INTERCEPTED THE G/S AND STARTED TO DSND ON IT. WHILE TALKING TO THE TWR; I GLANCED AT THE APCH PLATE AND SAW 24.7 DME ON ROBBI INTXN AND DID NOT SAY ANYTHING TO THE PLT FLYING BECAUSE WE WERE SHOWING 18.1 DME. AT THIS POINT; WE WERE GOING THROUGH PRE-LNDG CHKLISTS AND THE TWR CLRED US TO LAND ON 21L. AFTER COMPLETING THE CHKLIST; I LOOKED AT THE APCH PLATE AGAIN (COPY ENCLOSED) AND REALIZED THAT BY DSNDING ON THE G/S; WE HAD LEFT 5000' TOO EARLY. THE 18.1 DME WAS FROM THE ARPT AND NOT FROM CRL VOR WHICH IS HOW ROBBI IS DEFINED. OTHER THAN LEAVING ALT TOO SOON; THE APCH AND LNDG WAS NORMAL AND UNEVENTFUL. NOTHING WAS SAID TO US BY THE TWR OR APCH CTL ABOUT LEAVING 5000' BEFORE ROBBI. I BELIEVE THIS SITUATION COULD HAVE BEEN AVOIDED BY A MORE COMPLETE REVIEW OF THE APCH PROC WHEN THEY CHANGED RWYS ON US.