Lot of talk lately about what Harper will contribute this year. Unfortunately the answer is almost certaily very little. According to Baseball-Reference.com exactly 58 players have played 40 or more games as 19 year olds since 1901. About one every other year. Almost all of them were lousy. Only two were really good. Mell Ott in 1928 and Jimmie Foxx in 1927. They had OPS of 921 and 908, respectively. (I used OPS as my comparison - nothing's perfect).
Some really great players played as 19 year olds; Robin Yount, ARod, Pudge, Justin Upton, Beltre. All of them had OPS of less than 700 which is pretty weak.
The most recent decent season by a 19 year old was Renteira in '96 with an OPS of 757. Last year, an average starter put up OPS of about 780, so 757 isn't great.
So, it's been 84 years since a 19 year old has put up an OPS better than an average starter.
Mike Trout is interesting to consider. He was ranked ahead of Harper by Baseball America last year, his OPS last year was 672. His slash line was 220/281/390. That is awful. It's really hard to justify giving a starting spot to a guy performing like that.
Trout will probably be much better this year. Twenty year olds do much better. There have been 186 players with at least 40 games played at age 20. And a number of them were excellent. Mel Ott improved to OPS of 1.084 at 20, which is amazing. Some other great ones were ARod, Ted Williams, Al Kaline, Jimmie Foxx, Ruth, Frank Robinson and Mantle. Even here though, only 32 have EVER put up OPS of >780 at age 20. But, at least it's happened multiple times in living memory. Just in the last 20 or so years the following have had OPS >780 at age 20; ARod, Heyward, Griffey, Stanton, J Upton and Cabrera. I think Harper has a chance to be in that group...as a 20 year old.
I think it's important to understand how rare it would be for Harper to have a strong season this year. More likely, he will be in the Ian Desmond, Rick Ankiel range this year with OPS of 650 or so. That would actually be a big accomplishment, putting him in the top 10 all time seasons for a 19 year old.
Of course, I'd love see the 19 year list re-written with Harper at the top or even just below Ott and Foxx. But, it's really asking too much of him. Let's hope for a strong '13 from him.