Buy as young as you can afford, and look for players with strong white skills (some players with higher ratings are boosted by grey attributes).
I use to think this way too, but their rating may not reflect their ability for a single position. For instance, a 120% DC/DR will have aggression, speed, etc. to boost their ratings. But a pure DC at 110% may have higher white skills for DC. Last season, I noticed that my team conceded more when I play my higher rated DC/DR/DL instead of my lower rated DC.