Hey
You can start doing LST whenever you're ready, it can handle gentle tying at this young age or you could wait for it to get a little taller. But definitely do it while the plant is still flexible and able to bend, versus waiting too long and the stems get all woody and stiff.
With the topping you want several sets of true leaves before you begin, typically between 5-7 sets. So it looks like you need at least another 3 sets to open up.
But the lst you could do now if you want that since smooth bend.
Happy growing ✌️