It's noted a couple places that Bryce* will be only the 3rd 18 yr old to be assigned to the AFL. Out of curiosity, I sought out the other youngest players to be seen in the AFL and checked out how they did this past year. It's not a great comparison because none of them are BRYCE, but hey - it's something to look at on a rainy Thursday.
(I'm giving you an opinion of their play based on raw stats. When you consider their ages, they are performing better than the raw stats because they are all young for their league)
Fernando Maritnez - assigned in 2007 but didn't play, stuggled a bit in AAA, not quite major league ready in a couple of short stints
Carlos Triunfel - assigned in 2008, struggled in AA after breaking his leg and missing most of 2009
Jose Iglesias - assigned in 2009, did ok in AA, back again in the AFL this year.
Starlin Castro - assigned in 2008, will get ROY votes in 2010 after a nice 3/4 of a season.
They've all done decently when you've considered their age and they've all progressed at least acceptably so far. Just looking at these four though gives you an idea of what can happen though. Martinez did not progress quickly and may have hit a ceiling in AAA (though you can't be sure about that for a good 3 more years). Triunfel got injured. On the flip side, Castro is a top rookie in the majors a year and a half later. Right now, almost anything is possible for Bryce.
On a side note : It's nice to say Bryce Harper led the Nats instructional league in this or that, but without reporting the stats it's meaningless. He could have easily led the team in homers, RBIs or walks simply by getting up more than anyone else.
*YES I'm going to always call him "Bryce" or "Bryce Harper" and not "Harper". I'M HARPER. It's bad enough that we've gone through a decade of parents robbing my name of it's masculinity because they have to name their daughters traditional boy names (did you know George Stephanopoulos named his daugther "Elliot"? I mean, COME ON people. How long till I run into a little girl named Steve?) I am not going to lose my own name on my own blog.