פירוש על עבודה זרה 38:4

Generally, a student should not begin to issue legal instruction until they have reached the ripe old age of 40. Rava was an exception because there was no one older than him. In such a situation, he had no choice but to act as a master of instruction. I’ll add in here a personal note. I began to teach Talmud at the age of 27. There was no doubt that I was too young, but there was no one else willing to teach, so I took the job.