So let us say that if there were two minors that ran away together.
If one of them turns 18 after they have run away, could the 18 year old be arrested for kidnapping?
"Kidnapping is the unlawful taking away or transportation of a person against that person's will"
The crime would occur if the second person was taken by the first, the age of the perpetrator is immaterial to the commission of the crime but it will affect where and how it is prosecuted. The age of the victim will matter as the victim must be old enough to competently grant consent - if they are too young to do so then it will be a kidnapping.
On the face of it "running away together" does not imply an unwillingness.
Note also that kidnapping is the crime of "taking away"; holding a person against their will is the different crime of false imprisonment.