Is a 5.9 inch penis average or small for a 14 year old?
yes it is average.
A 4 inch penis is on the small side with the average length being 5 inches but as for it being too small for an 18 year old, only she can know, because all women are different sizes down there and like different things. the point is if the said 18 year old is in fact not satisfied there are other option like oral sex or mutual masturbation to look into. Don't be discouraged!