You wrote that no assistance service is available for kids/adults 18 and over. I think you are wrong. Someone over 18 would not be an UMNR - theyre not minors, but they could very well be so-called MAAS - meet and assist. This kind of assistance is somewhat loser, but someone will meet the kid, or in this case, young adult, at the jetway. This would be good when f.ex. language problems are involved.

By the way, I think I remember having an 18 year old UM with CO. Probably some parent thought that was best when flying internationally. The person picking her up was also 18. And listed as "boyfriend" under realation on the form... Way off-topic thou.

But have in mind, anyone can be a MAAS.