AN EARLY TURN ON THE MOUNTAIN 5 DEP FROM SEATTLE; WA; IS NOTED BY BUT DOES NOT CAUSE ATC ANY DIFFICULTY.

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ON THE INITIAL CLBOUT; NBOUND; MOUNTAIN 5 DEP (FO FLYING); WE MISTAKENLY TURNED TO THE E EARLIER THAN THE REQUIRED 8 DME. DURING THE TURN; WE RECOGNIZED THE ERROR AND BEGAN TO CORRECT BACK; AND SIMULTANEOUSLY; DEP CTL POINTED OUT THE EARLY TURN AND GAVE US VECTORS TOWARD OUR ROUTING. NO APPARENT TFC CONFLICT AROSE. WE ACKNOWLEDGED AND APOLOGIZED; AND DEP SAID NO PROB. DURING THE TKOF BRIEFING; WE REVIEWED THE DEP TOGETHER; AND BOTH OVERLOOKED THE 8 DME RESTR. I BELIEVE WE WERE FOCUSED ON THE 4000 FT MINIMUM HIGHLIGHTED RESTR; AND THAT WAS HOW WE FLEW THE TURN. WE NEED TO BE MORE THOROUGH IN THE DEP REVIEW AND MORE ATTENTIVE WHILE FLYING. SUPPLEMENTAL INFO FROM 563827: ENTERED THE WX AT APPROX 4500 FT ON DEP LEG; GLANCED BACK AT THE DEP PAGE AND SAW THE R TURN TO 070 DEGS WHEN ABOVE 4000 FT. BEGAN THE TURN AT APPROX 6 DME; AFTER APPROX 60 DEGS OF TURN AND CLBING BACK TO VMC GLANCED BACK TO THE PROC PLATE AND SAW THE 8D NEXT TO THE TURN NOTE AND MADE AN IMMEDIATE CORRECTION BACK TO TRACK. DESPITE REVIEWING THE DEP PROC (TWICE); THE SIMPLE DISTR OF AN IMC DECK ONLY 500 FT THICK CAN CAUSE YOUR BRAIN TO SKIP A BEAT WHEN THAT EVENT IS AT THE SAME ALT AS A CUE ON A PROC.