Board Thread:Naruto Discussions/@comment-24793983-20140921030415/@comment-1018594-20140921064111

It's assumed that Rin's spirit stopped aging when she died, and it would be really creepy to see a 13-year-old Rin (assuming her and Obito were the same age during Kakashi Gaiden) and 31-year-old (assuming him and Kakashi are the same age), scarred-along-the-entire-right-half-of-his-body, barefoot, gray-haired Obito in a loving embrace, with Obito crying as Rin assured him that she was watching over him the entire time.

It also goes without saying that Obito telling Kakashi not to join them any time soon is the equivalent of a sock on the door. That idea becomes so, SO much more disturbing if you factor in the age gap.