Emperor Pilaf Saga
Main article: Emperor Pilaf Saga Initially, Roshi proposes the Phoenix bestow a young Goku with immortality as a reward for escorting Turtle to the sea, but this is cut short by Turtle's reminder to Roshi that the Immortal Phoenix ironically died from being fed "tainted bird seed". Master Roshi instead gives him the Flying Nimbus as a gift for his philanthropy.
The Immortal Phoenix is allegedly capable of granting immortality (which curiously is one of the leading causes for why many villains in the series seek the seven Dragon Balls). Presumably, this bird was the reason why Roshi does not die of old age.