The actors playing Harry, Ron, and Hermione in Harry Potter and The Deathly Hallows are most certainly famous, but almost certainly not because they are talented. They were cast in that movie nearly 10 years ago when their average age was 11. No doubt talent played a role in that selection but acting talent at the age of 11 is no predictor of talent at age 20. The fact that they are in Deathly Hallows is statistically independent of how talented they are.It is probably true that Daniel Radcliffe was no more talented an actor at age 11 than the typical 11-year-old actor wannabe, but he has gone down a different path over the past 9 years. At age 20, it is unlikely that he has the same relative talent as the 11-year-olds he beat out for the Harry Potter job. In other words, it is because Radcliffe was selected that he became more talented.
If Radcliffe becomes immensely successful in other roles, we can probably attribute this success to his selection to be Harry Potter at age 11. But, that does not allow us to deduce that fame alone gave him the chance at success (as Ely notes in his original post). There is still a role for talent.
Just as the commenters on the original post point out, Radcliffe was given a great opportunity to learn by doing on the set of Harry Potter. As a result, he is much more talented today than he would have been if he hadn't gotten the role. This acquired talent undoubtedly has a payoff, and is likely be the reason you'll see Radcliffe be more successful at acting than the randomly selected 20-year-old.