* Immature: Not yet mature, lacking in maturity.
* Premature: Happening or done too early, before the usual or proper time.
* Overmature: Mature beyond the usual or desirable stage.
* Supermature: Exceptionally mature, exceeding the normal level of maturity.
* Co-mature: Maturing at the same time or rate.
The best prefix to use depends on the specific meaning you want to convey.