This "experience sexual orgasm" slang meaning of come (perhaps originally come off) is attested by 1650, in "Walking In A Meadowe Greene," in a folio of "loose songs" collected by Bishop Percy.
They lay soe close together,
they made me much to wonder;
I knew not which was wether,
vntill I saw her vnder.
then off he came & blusht for shame
soe soone that he had endit;
yet still shee lyes, & to him cryes,
"Once More, & none can mend it."
It probably is older and disguised in puns, e.g. "I come, I come, sweet death, rock me a-sleep!" ["Nashe His Dildo," 1590s]
As a noun meaning "semen or other product of orgasm" come is attested by the 1920s. Spelling variant “cum” by 1973.