Well I'm not familiar with growing coleus from seeds (I usually do it from cuttings), but I see no reason why you couldn't do it now, if you have enough light to sustain them until time to plant outside. I think the only thing against doing it earlier is that they might get too big for the amount of light available and thus get leggy. But I would think now is NOT too early.
I would start now. They take a little while to get going. Coleus are warm germinators so there is nothing special about planting the seed besides keep them moist and warm. Once they come up, give them enough light so they don't get leggy. I grew coleus from seed years ago. Not all plants were impressive but there were a few nice ones. I also found the seed was short lived. Anyway, I couldn't get the seeds to germinate the following year. I collected the seed myself.