I have introduced a take off fairly early (depending on a lot of factors). The times I have is when I saw the student hit a plateau or took a step backward; basically getting frustrated in their lack of progress. I’ll bring them down to the ground and have them practice taxi as a change of pace.

A few times I was able to get students to taxi faster and faster straight down the runway after a few minutes of practice. when I see the plane has hit flying speed I say “bump up just a little” and it takes off. Of course I’ve got my fingers on the sticks of the master transmitter just like I were flying it and will release the trainer switch moments after the wheels leave the ground, but hey they took off and it does wonders for their psyche.

I always introduce takes offs before landings. The question of how early depends on the student.