When Bryce Harper was a kid, he didn't have an SNES. That was old technology. When he came of age, he was probably a 5-year-old with a Playstation. That's how young he is. Sure, you can list his exact age, but where's the fun in that? Point is, he's young.
But he could start for the Nationals as soon as this season. He could break camp with the team. Pete Kerzel of MASN Sports lists five things Harper has to do to make the 25-man roster, and suggests that the decision will be a tug-of-war between manager Davey Johnson and GM Mike Rizzo:
Both sides have some history on the side of their arguments. As the New York Mets' manager in 1984, Johnson worked hard to convince then-GM Frank Cashen that a 19-year-old flame-throwing pitcher named Dwight Gooden was good enough to advance from Single-A to the bigs. Johnson won, Gooden dominated and the Mets were World Series champs by 1986. Rizzo's got a long history as a talent evaluator and scout and knows a thing or two about developing players.
Kerzel's list of five things is one part physical, and four parts mental. Harper has the five tools, and he can probably hit right now, but not being a jackass is an underrated sixth tool. Whether or not he has that one is still an open question.