First of all, I would wait until last chance of frost has passed. In your area mid to late May would be a safe time, so let another 3-4 weeks go by before you attempt.

At that time, they will be 5-6 weeks old and thus be large enough to establish themselves without problems. Generally I would say as minimum they should have 2-3 sets of true leaves and have thicked up a bit. Anything less than 4 weeks is a bit premature for impatiens, but probably possible if you had to, but do wait for May to come around.

I would transplant the seedlings to larger trays (cell packs, peat pots, 2.5" round plastic pots, plastic drinking glasses, etc) when the leaves of the seedlings are touching each other. Then keep them growing indoors until after last chance of frost has passed in May.

Start now by fertilizing with a diluted fertilizer. If you use miracle grow 15-30-15 then use 1/2 tsp per gallon of water.

Good luck.

Thanks a lot Ken. Great info you provide for everyone here.