Using the Nationals affiliates as a guide ... a good rule of thumb is a HS player spends his draft year in the GCL (~18 yrs old).
Ideally a start in Vermont in year two with the potential of a late season bump to Hagerstown if the player is a hitter (makes sense to give pitchers more time, espeically a young one). (~19 yrs old)
It's in that second full season (or third year) that the jump should take place for a first round draft pick. The premium guys may go to Potomac to start year three but it's also not unusual for them to start year three in Hagerstown and work their way to Potomac mid-season. If they are being cautious, a full season in Hagerstown is not out of the question (~20 yrs old)
In the fourth year, if they didn't make it Potomac, they should be there to start. And the goal is to be in Harrisburg by the end of the season. (~21 yrs old)
At that point, they still have one season until they would need to be added to the 40-man or be exposed to the Rule 5 draft.