I pictured Anakin to look a little older than Luke, mid-to-late 20s, maybe early 30s. Dark features, tall, masculine, strongly built. I imagined him to be an alpha-male type, at least in appearance, but be a charismatic and caring person, something like Starkiller's personality when he turned good.

I never imagined him to look the same age or younger than Luke. And although it makes sense that he would have light colored hair like Luke, I always imagined a young Vader to have dark features.

One more thing, I never liked the reveal of Vader under the mask in ROTJ. I assumed his voice was affected somewhat by his respiratory machine but even so, a British accent would not disappear. I also thought his face was going to be way more scarred up as hinted in ESB, but here he's just a pale bald older Brit, with or without eyebrows.