First of all leaves turn yellow for all differently. Reasons temp of your room. temprature to your plants or low humidity. Between 20-25 but high temp overwartering -underwatering . Some strains like shark from seed when there young they will turn yellowish at some part of the veg stage my advise to you would be daggle your hydrometer from your light 16" from the bottom of your light bulb to the tip of your plant then that will give you a rough idea what's going on with the temprature and humidty oft the plant
you got pics??? It could be many things, we need to hear more about your grow. But like Hamster there said, the sick plants section would be a better place to post. Welcome to MP