Exponential growth also pushes down the time when you need to become a mentor. If the field was totally stable (retirement rate = graduation rate, and everyone retires after 40 years in the field, greatly simplified model), then each new joiner could be paired with a mentor of 20 years experience, and only needs to become a mentor after 20 years. But if the field is growing exponentially, the age drops significantly. I'm sure someone could calculate this; it's almost like the inverse of the retirement age, population change, and social security question