Here I present Uncanny X-Men #182 from 1984 where Rogue's identifies herself as 18:
The Savage Land arc takes place in Uncanny X-Men #274 in 1991, so 7 years later in real time.
However in comics time: "The general rule of thumb is for about every five years of comics, one year of time has passed in the Marvel Universe (with a few notable exceptions). So, while almost 60 years have passed in the real world since the first appearance of Spider-Man, only about 10-15 years have passed in the story."
So by that estimate about 1.4 years have passed since Rogue told us she is 18 to where she meets Magneto in the Savage Land, meaning Rogue is approximately 19.5 when Magneto (who is the same age as Charles Xavier) is getting all hot and bothered over the mostly naked girl who is just waking up from nearly dying.
