When I started to learn to do pruning as defoliation was known back in those days, I started on a Bonsai tree I bought on the spur of the moment at some street corner vendor, and from there I had to buy a book to teach me all all things from training devices to force the plant to be shaped over time as it grew, I had to learn to do pruning and once or even twice a year root pruning also had to take place. It took me a couple of little costly trees to learn some very important lessons like sterilizing the cutters between cuts, having a clean working area, etc... From there I got the "basics" I guess you could say down. So from that point I begin to try pruning other things like fruit trees and then other plants, thankfully I only had one very expensive little tree to start with so I wasn't in a rush to start getting pruning.
On that note I have had success with pruning from an early age on up... to an older plant, but it should also be noted I've learned over the years what to do and not to do, for example when pruning a younger plant. You can really better control how the leaves are going to grow and where, you get to be in 100% control , and it is important to remember every leaf a plant makes is its power source, every leaf you cut off that plant made you just wasted energy and threw it away, so yes controlling the leaves from an early age is best as the plant is vigorous in its lifecycle and is at a stage of wanting to really make a lot of leaves and grow and grow, so cutting here and there to shape it and control it doesn't really do much for any harm being done, as you stay on top of it
Once a plant is older those BIG leaves everyone thinks needs to be cut off are established powerhouses for that plant, cutting them off is like saying you have two legs, so we will just cut one off and you'll still have one so you'll be okay. It doesn't work that way, bending a big leaf out of the way is much better than cutting off such a powerhouse. Its when you start getting in there and cutting off the big leaves that the plant is now shocked, has to stop and concentrate on making those smaller leaves bigger to make up for the lost energy production of one big leaf, thus hurting the growth rate of the plant or the bud. I am a firm believer there is two kinds of stress for a plant, a healthy stress like wind and air flow blowing it around a bit, and bad stress like a pest attack. A part of the secret to pruning in my opinion is learning to control the stress of removing leaves to help the plant by creating healthy stress. If a set of leaves are not getting light way at the bottom, you have to know if it is better to remove leaves from the top and let more light into the bottom or if you should cut those bottom leaves to allow for more bud production at the top.
As any experienced grower knows you want the bud on top, closer to the light as anything below will only be popcorn, by controlling defoliation early on you could grow a top canopy and concentrate all the bud growth to the very tops, with multiple tops all equal heights and push all that energy from the would be popcorn buds all up to making huge top cola's on every branch you allowed to grow at equal height, thus increasing yield and potency as I've heard it also claimed. Hence why one person can claim pruning hurt their yields and the plants, even perhaps killing the plant in some cases, while the next person will claim it increased yields... all because of how you defoliate a plant, what stage the plant is at, how you go about it and even the strain and the plant's lifecycle all matters. If a plant never experiences wind and suddenly it does, the plant will fold under the stress of the wind, but if the plant always was exposed to wind the wind would be a positive stress to create a stronger plant.
