That is like asking, "[w]hat age did you smoke pot" (hagar 11).
You will get a numerous amount of answers. Some people do it EARLY, some wait till the 4th node, mainly because most believe the plant is in the full swing of veg and has maturied out of he seedling stage.
Some people (during flowering) feed the entire term of the lief cycle, some flush 2 weeks prior. It is all preferential tot he gardener's will.
I personally do both.. if i beileve that plant needs a little extra incentive(LOL) i maximize P, and K (specially in satavs.)