When I passed my test at the tender age of 17, you could accompany learners immediately.
The law has changed since those far off days - you have to be at least 21 years old and have had a full licence for three years (for that type of vehicle).
There was also a time when anyone could be an instructor so long as they had passed the basic driving test. These days there are lots of exams to pass and continued testing.
A video from The Rank Organisation dated December 1959 looking at the driving test at the time: