Age for LST?

As long as the stem has not become rigid, you can still LST. Exactly when? I'd work backward from a target canopy with 2.3-2.5 colas per sq ft. This will maximize yield without adding extra mouths to feed, which means you get a higher percentage of high quality buds vs larf. It's a good compromise of light penetration and airflow, yet still fully filling the area. More colas per sq ft doesn't add to yield and only causes greater percentage of larf. Genetics matter.. some will react differently than others. If it's a plant with tiny buds, maybe a few more colas per sq ft could be a benefit. Anyway, this can guide any LST or topping... How many primary branches do you want? Will you top those or is that the end of the training? Now you know how many branches you want to originate from the early LST. Anything on or near the bend will fall behind - even if you train it to be the same height as the others, it'll always be less. So, pruny off anything on or below the bend you create. Start bending as low as you can without stressing roots -- you don't want to be yanking the stem around in the soil. Move fingers up a cm or more and then bend again.. bend it in 2-3 places to form a bend that mostly stays in position. Anchor the bottom of the trunk to the opposite wall of pot relative to bend. Almost no tension but does hold it steady in place. Now, you tied down the top of the plant to get it mostly level and so that it retains that shape going forward. Anchor preserves roots and the tie down up top you continually move out futher on the plant, or add additional tie-downs to keep the shape until you have enough primary branches to form your end-target canopy. Anything facing directly down should be pruned. Anything resting on the substrate should be pruned. Otherwise retain as much foliage as possible. You can bend between the branches (45-degree angle) and both branches can go vertically, but i'd only do this if you intend to have a massive plant. You'l have more branches than you know what to do with very easily. so let them take the most direct route. Bend the plant down so that 1 is vertical and the other of that set of symmetrical growth that is facing downward is pruned off. You'll more often run out of space if you keep everything. It can quickly become a congested mess. Manipulating apical dominance is a tool. Use it anyway you see fit. Don't worry about what arbitrary category it falls under whether lst or topping or anything other extra words describing the same cause and effect.

Generally people go off what "node" the plant is at. So around the 4th or 5th Node. If you are just going off days, I would say around day 14-21 is a good time.