1- “Every complete tzaddik is a baal teshuva. If he is a genuine tzaddik he is constantly making a cheshbon hanefesh and doing teshuva.”
So, how do you learn Pshat in that Gemara?
2- Who determines at what age who can become great?
Also, aybe the development of great ones needs the atmosphere, and challenge provided by the 999 others! Finally, Kollel is limud Torah Lishma…… anyone should be allowed if he can and wants. It may even be more LISHMA for the one who plans to work later on than for the one who wants to learn and then get a job in Klall positions……