^For your first feeding, that all depends on the size of your pot, vs. the size of your plant. It doesn't depend on age, because I've seen plants at 2 weeks, that are as big as some I've seen at 2 months. For example,...if your plant is 6" tall, and about 6" wide, it would be close to needing fed if it were in a 1 gallon pot, which is also about 6" wide and 6" tall. My reasoning for that, is that the roots are also about 6' wide and 6" tall(for arguement's sake), reflecting the growth above ground, telling me that most of the medium has been occupied by roots, which'll depete the soil in a week or so, afterwards.(meaning that it's time to start feeding)
Now, if you have that same plant in a 5 gallon pot, the soil will sustain it much longer, until the plant is much bigger, reguardless of age. So, in the situation above, you could either start feeding lightly, or transplant, doing the same thing, basically. Or, you could give it a light feeding, replenishing that old soil, then transplant, giving it the best of both worlds. I do this.
Feeding, is a tough thing to describe to a new grower, due to the many variables. Generally, it takes a fert burn, or two, to convince the new guy that he's getting a little carried away, because most guys seems to think that plants are like people....more food = fatter and faster growing.lol That's not true. They eat according to how comfortable they are in their environment, and most importantly...how much light they are processing. The more light they get, the faster they process, the more nutes they require(use). To speed things up, you add more light first, then satisfy their nute requirements. Ooops, sorry, I kinda drifted off track....lol