At what age did Rav Yosef become blind?
Burton G. Kaplan
I don't know - and I don't think that Chazal mention it. What we do know however, is that he was not born blind, and that he became blind of choice.
Be'Virchas Kol Tuv,
I believe it was Rav Sheishess who became blind by choice.
Rav Yosef could have become blind at the time when he became sick, forgot his learning and his student Abaye
1) Rashi in Menachos 110a (DH Simi At) writes that some say that Rav did not lift up his eyes due to modesty. Rabeinu Gershom there (Menachos 109b, printed in some editions on 110a) writes that Rav Yosef and Rav Sheshes, who were Talmidim of Rav, also wanted to adopt this practice, and they became blind as a result. The Ran in Kidushin (near the beginning of 13a in the pages of the Rif) writes that Rav Yosef blinded himself for this reason.
2) The Seder ha'Doros, in his entry on Rav Yosef, cites the Mekor Chayim at the end of Parshas Shelach, who says that Rav Yosef sat in a dark place for 40 days and then suddenly looked at shining marble stones in order to make himself blind.
3) According to these sources, both Rav Yosef and Rav Sheshes became blind out of choice. (See also Rashi to Nidah 24a, DH Simi At.)
I found an earlier source for the above in a Sefer called Sha'arei Teshuvah (#178), a collection of Teshuvos ha'Ge'onim, responsa from the Ge'onim, an even earlier source than Rabeinu Gershom.
He writes that Rav Yosef and Rav Sheshes tried to emulate Rav, but they were not able to keep up this practice until they blinded their eyes.