Depends what gave it that 18+ rating. There are slightly different standards of decency we've come to accept. For example, England finds certain words offensive that we Americans would find the same words funny and/or playful. I think our tolerances to the concept of violence might be different as well.
Also, the rating "PG-13" (as well as the ratings "G," "PG," "R," and "NC-17,") belongs to the MPAA, so Funimation can't rate anything using those symbols themselves. They need the MPAA to rate that for them. If I recall right, the other NTE movies ave been rated "PG-13" by the MPAA, so this should also be rated by that same organization. Other anime's released in the US simply gave their approximate equivalent rating to a movie or TV show, resulting in "13+" symbols to indicate how appropriate the distributors thought the media was for children. These are used because the media was not rated by the MPAA.
All that being said, Anno's been known to go off the rails for his final theatrical Eva installments. The original OA show coasts on a generalized "PG-13"/"TV-14" aesthetic throughout, but I've absolutely no idea what The End of Evangelion would receive if it were rated by the MPAA. I tend to think of it as one of the best "NC-17" movies never rated by the MPAA.
Eva Q does seem to be exploring the blurry areas between MPAA's "PG-13" and "R" ratings. I don't know if Shin Eva will be more explicit in its content or not.