Berakhot 28A

דלמא מעברין לך. אמר לה: לשתמש אינש יומא חדא בכסא דמוקרא, ולמחר ליתבר. אמרה ליה: לית לך חיורתא. ההוא יומא בר תמני סרי שני הוה, אתרחיש ליה ניסא ואהדרו ליה תמני סרי דרי חיורתא. היינו דקאמר רבי אלעזר בן עזריה: הרי אני כבן שבעים שנה. ולא ״בן שבעים שנה״.

There is room for concern. Perhaps they will remove you from office just as they removed Rabban Gamliel. He said to her, based on the folk saying: Let a person use an expensive goblet one day and let it break tomorrow. In other words, one should take advantage of an opportunity that presents itself and he need not concern himself whether or not it will last. She said to him: You have no white hair, and it is inappropriate for one so young to head the rabbis. The Talmud relates: That day, he was 18 years old, a miracle transpired for him and 18 rows of hair turned white. The Talmud comments: That explains that which R' Elazar ben Azarya said: I am as one who is 70 years old and he did not say: I am 70 years old, because he looked older than he actually was.

תנא אותו היום, סלקוהו לשומר הפתח ונתנה להם רשות לתלמידים ליכנס. שהיה רבן גמליאל מכריז ואומר: כל תלמיד שאין תוכו כברו, לא יכנס לבית המדרש.

It was taught: On that day that they removed Rabban Gamliel from his position and appointed R' Elazar ben Azarya in his place, there was also a fundamental change in the general approach of the study hall as they dismissed the guard at the door and permission was granted to the students to enter. Instead of Rabban Gamliel’s selective approach that asserted that the students must be screened before accepting them into the study hall, the new approach asserted that anyone who seeks to study should be given opportunity to do so. As Rabban Gamliel would proclaim and say: Any student whose inside, his thoughts and feelings, are not like his outside, i.e., his conduct and his character traits are lacking, will not enter the study hall.