It's not necessarily too early providing they're kept warm with plenty of light.
CLER, is there any chance of posting a photo? In the meantime, what size are they? Do they have real leaves? Yellowing of leaves on a young plant can be a sign of overwatering but also of insufficent light. Or even both.
Am I right in thinking yellow leaves can sometimes be a lack of Nitrogen in the soil?
They just need moving on into larger pots, as Mattbeer87 says it's a lack on nutrients in the compost and potting on is the answer, they'll soon green up again.
If there's a lack of anything, it's more likely magnesium. If repotting doesn't sort it, try spraying the foliage with Epsom salts.