if ur growing in a soil medium... and it's decent soil, many people will say that nutes before 3 weeks are unnescessary. personally i don't ever opt for the super expensive soil, so i'm of the opinion that its never to early to begin a very light nute program. like 1/5th strength root development fertilizer from the time they are seedlings. and then a 20-20-20ish (miracle grow for tomatoes, ie at about 1/5th strength going into week two. If you are fertilizing lightly enough, its not going to hurt the plant. So i'd rather have these nutes available to a plant in case it is ready to use them.
other opinions welcome, i'm certainly still learning.
I started feeding my plants half strength at 3 weeks...got SLIGHT burn on full strength but half was fine every other watering - just put me bushes into flower!