It would be best to wait untill they are at least one month old or have 4 sets of fan leaves before you start to use fertilizers. Some people start after the first full fan leaf is developed (5 to 13 leaves) but they start with a weakened mix somewhere like around a 1/4 of the recommended mix. Others dont fert untill flowering( as I do) unless the need is there. Either way start fertilizing with 1/4 to 1/2 strenth mix building up to 100% to 250% recommended mix, when you start to use ferts.