This subject has been broached a lot lately, so with the help of Brian Oliver, here’s a chart of ideal ages for each level.
16-18 — DSL (Yes, occasionally some 19-20-y.o.’s will make it north of FL, but that’s usually pitchers)
18-19 — GCL (HSers and international guys with a year or two experience)
20-22 — NYPL (JuCo/college draft picks and guys who have spent a year at GCL)
21-23 — Low-A (Mostly guys who spent previous season at NYPL, occasional leapfrog from GCL)
22-24 — High-A
23-25 — Double-A
24-26 — Triple-A
Baseball America also recently did a story about the median ages of the full-season leagues, which supports these rules of thumb.
Most of the elite draft picks have been starting at Hagerstown under the Rizzo front office (2009-present), which is also a byproduct of the majority of them being college juniors or seniors.