Growth rates vary from water to water and geographic location.
Within a specific location you could have a lake on one side of a road that has a population of 12" crappies that are 3-4 years old while across the road from that lake have another with 12 inch crappies that are 5-6 years old. Some lakes have a forage base that allows crappies to grow like wild fire, others may be forage poor and crappies grow only marginally year to year.
The further south you go in the United States, the faster the growth rates. Here in Minnesota we see ice on the water for 4-5 months each year. The ideal temp for crappie growth with a good forage base may only be available for crappies to actually gain weight and length could only be 4 months as opposed to maybe 8-9 months in Georgia. The longer the water temps stays at or slightly above those where optimal growth occurs, the faster the fish show length.
Most 12 inch fish in Minnesota are getting up there. I'll hazard a guess that a 12 incher up here is at least 6 years old in most instances, maybe older.