Luke Skywalker is, without doubt, one of the most popular characters in the history of film. Mark Hamill thinks the reason that Luke resonated with so many is actually quite simple. It wasn't that Luke was a particularly special character in any way. Rather, it was the opposite. The fact that Luke Skywalker was such a normal character made him more accessible, especially to the young children that made him into a hero.
I wasn't 6'2", you know, a perfectly fit specimen. You're right, I was kind of a geek, with no discernible skills. But I thought that was reassuring. ... Now, the kids might be a little wary of Han Solo. He's swaggery, and he's a rogue and a pirate and all that. And the princess is royalty, and Darth Vader is scary. Of all the characters, Luke is the most accessible in terms of a 9-year-old going, 'Oh yeah, I can I hang out with him and not be intimidated.
The whole basis of Luke Skywalker's character in Star Wars is that he's very much an average character. While he has a fairly impressive backstory, even Luke himself is unaware of it at the time. Skywalker is the definition of normal, but, eventually, he becomes somebody special. Clearly, these sorts of ideas could appeal to a young child.
Mark Hamill made the comments as a guest on the Nerdist Podcast, where he also does a stellar impression of Harrison Ford that needs to be heard to be believed.
While that feeling that Luke Skywalker was the most approachable character probably hasn't been a conscious thought from most of the young children who have watched Star Wars over the past 40 years, it probably is something that has been subconsciously understood. While much of that is designed in the writing of the character, it also requires the right actor to pull it off. If Mark Hamill had been a different sort of actor, if he had actually been a 6' 2" perfectly fit specimen, no script in the world would have made him feel like the same character that we actually got.
It will be interesting to see just how accessible Luke Skywalker feels when we see him in Star Wars: Episode VIII. We know we'll be seeing a great deal more of him in that film. The life that Luke has led for the last 20 years or so is probably not quite as accessible as what he went through growing up. Will a nine-year-old watching Episode VIII have the same feeling about Luke? Probably not, though it will be interesting to see how much of the farm boy is still left in the character now.