Wait till you have 3 or 4 solid tips showing with 5 and 6 coming nicely before adding nutes.. I know some guys that will leave them till the bottom leaves start to yellow.. then clip em off and start with nutes then.. But I feel this is unnecessary stress on the plant.. Best way to tell is by how big they are.. not how old! Every strain will be a little bit different.. so just do what feels best for your situation :-)