Ballparking fish in the northern half of the U.S., not considering productivity and all the other variables, Black crappie at 9" would probably be around 5 years old, at 17" 12-15 years old.
Same circumstances for a white crappie, probably 3 years old for the 9"er, 11-13 years old for the 17".
Very few fish reach 17"...hard to avoid anglers and predators for that, fast-growing fish often die younger, and too many fish (or fish in certain lakes) just reach environmental limitations by that time. Quite a treasure catching crappies over 14.5", it's just hard to find them that big. Not impossible, just very challenging.