Berachos 28a - Tefillas Arvis R'shus Oy Chovah - Part II

The gemara says that Rabbi Elazar ben Azaryah was considering not accepting the position of Nasi because of his age - he was only eighteen! The gemara says: "That day a miracle occurred to him, and eighteen rows of white hair were placed in his beard. And, this explains what Rabbi Elazar ben Azaryah said (on 12b), 'I am like a seventy year old,' and not 'I am a seventy year old.'"
This gemara seemingly contradicts a Rambam in his Pirush Hamishnayos. The Rambam, in Pirush Hamishnayos on the last mishna of the first chapter of Berachos says: "The reason why Rabbi Elazar ben Azaryah said, 'I am like a seventy year old,' and he didn't say, 'I am a seventy year old,' is because he was really a young man. However, he learned a lot, day and night, until he was weakened, and he prematurely aged to be like a seventy year old. And he agreed to the beginning of the aging, as the gemara explains."
The Rambam - by attributing Rabbi Elazar ben Azaryah's aging to exertion in Torah learning - seems to discount the miracle that the gemara relates, and says a rational explanation that argues with the gemara's explanation.
However, we can explain like this: The Rambam was bothered by a question. The gemara seems to say that Rabbi Elazar ben Azaryah looked like a seventy year old, because he (miraculously) had eighteen rows of white hair added to his beard. This implies that the rest of his beard remained its' original color. If that's so, Rabbi Elazar ben Azaryah didn't look like a seventy year old - has anyone ever seen a seventy year old with a striped beard? Therefore, the Rambam understood that the gemara must mean as follows: When Rabbi Elazar ben Azaryah was eighteen this miracle happened. Rabbi Elazar ben Azaryah, because of the miracle, agreed to accept the post of Nasi. The post of Nasi carries much more responsibility than Rabbi Elazar ben Azaryah was used to (see Maharitz Chiyus 27b in the name of R' Yonasan Eibischitz), but he wasn't fazed, and willingly accepted the post. Those extra duties prematurely aged him, so much so that a while later he was able to say, "I am like a seventy year old," because by then the rest of his hair had turned white. (That's what the Rambam means when he says, "And he agreed to the beginning of the aging, as the gemara explains." That means, that he agreed to accept the post of Nasi, which carried all the extra exertion in Torah matters along with it.)