When to Spar, etc.

This is a most difficult question. It is driven by several factors which start with the individual. Each individual is unique and Sensei must monitor those new to the martial arts closely so they know when that individual is "ready" to start applying their new knowledge in a controlled drill/sparring environment.

Pushing someone to fast into the dangerous and difficult physical contact can have adverse effects both on the individual and on the dojo. This brings back up the importance of relationship development in a difficult and dangerous practice.

Most martial artists and most martial arts instruction tend to put folks into that environment way to early. Long before they have encoded into their brains actual martial techniques. I attribute this to the sportification of martial arts. In the excitement of competition and winning trophies and accolades we tend to forget fundamental principles that make martial practices work and simply instruct in a few simple boxing/kicking techniques to get "points" and "win."

In my dojo, in my instruction, I would not allow anyone to participate in free sparring until the stage/level of "Go-kyu." It was simple, any practitioner who wants to actually learn martial arts, i.e. in my case karate, must establish a foundation comprised of fundamentals with principles or it will not work and be merely "fighting/brawling." This is why sport tends to NOT be martial arts or karate regardless of beliefs and instructions to the contrary.

The Sensei-deshi | Sempai-Kohai relationship is either a detriment or benefit to a practitioner where forward movement is dependent on how that is applied in instruction.

Note: There are many ways to train realistically so you learn proper application of martial techniques. Sparring both free style and drill style are only a small part.