As far as condition of asphalt, it is too late for help from sealcoating when your asphalt begins to have multiple large cracks and crumbling asphalt. Sealing this type of damaged asphalt will not save it from needing to be replaced.  Seasonally speaking, sealing your driveway in the winter can be dangerous because you must be careful that the sealer does not freeze when it is applied to the driveway. Thus, the best time to seal your driveway is typically in the spring, summer or fall.