jaskier is immortal because he’s a trans man who asked the wrong mage for transitional magic and did not realize in time what “you will be, truly and in every way, to all eyes for all time, a young man” actually meant

other unforeseen side effects of this magic include 

• a complete inability to hide (since anyone who perceives him visually will immediately know he’s a young man, even if the only visual data they receive are his shadow on the wall or movement out of the corner of their eye)
• not necessarily getting taken seriously–it’s a pretty brief step from “young man” to “callow youth” in the minds of many
• reading to animals as a young male of their species, which can make springtime a little ridiculous or, in the case of large ungulates who do not wish to have any competition for the females of the herd, outright dangerous. (People assume that the bull in “His Thund’rous Hooves” is a playful metaphor for an angry husband, and it is, but it didn’t start out that way.)

Bulls Hate Him! Local Bard Does NOT Want Your Woman, Ferdinand

Jaskier: Geralt did indeed drive off the bull without hurting it at all. I left out the part where he was laughing hysterically the whole time.

Geralt: I do not “laugh hysterically.”

Jasker: Right. I suppose you were doubled over and unable to cast magic because of the sheer existential terror of the whole scenario?

yes, hello? this.