I went back and looked at some of the articles to get an idea of when great-grandpa changed to person of interest.
http://www.eastidahonews.com/2015/07/sheriff-family-friend-not-a-suspect-in-deorr-kunz-case/
July 27th
"Bowerman said
Reinwand, similar to Mitchell and Kunz, are persons of interest in this case because they were at the scene. However, at this time, neither Reinwand, Mitchell or Kunz are suspects in the missing persons case."
http://www.ktvb.com/story/news/local/idaho/2015/07/28/lemhi-search-deorr-kuna-persons-interest/30799181/
July 28th
"Bowerman also confirmed that
Reinwand, as well as the boy's parents Jessica Mitchell and DeOrr Kunz are persons of interest in the case because they were at the scene. They are not being called suspects at this time."
Starting on July 31st, he began calling all four of them persons of interest:
http://www.kboi2.com/news/local/DeOrr-Kunz-Missing-Idaho-320303121.html
July 31st
"Bowerman said
everyone at the campsite is considered a person of interest in the case, however, none of them are suspects -- this includes the parents, the child's great-grandfather and family friend Isaac Reinwand."
http://www.eastidahonews.com/2015/07/parents-of-deorr-kunz-we-pray-our-lil-man-will-be-found/
July 31st
Earlier this week, Bowerman told EastIdahoNews.com that
DeOrrs parents, great-grandfather and Reinwand are considered persons of interest because they were all at the campsite but, at this point, none of them are considered suspects.
On August 18th, he revealed that all four of them took polygraphs, which tells me that great-grandpa can't be too mentally deficient (although apparently IR took one also and we have seen firsthand how mentally aware he is...)
http://www.eastidahonews.com/2015/08/lemhi-sheriff-opens-up-about-deorr-kunz-case/
August 18th
All four have voluntarily taken polygraphs and right now theyve been turned over to the FBI, Bowerman said.
Not sure this tells us anything really. Just that sometime between the 27th and the 31st, the sheriff started to include great-grandpa as a person of interest. Also, the later articles also stop commenting on his declining physical and mental health. Perhaps his health issues were somewhat exaggerated at the beginning? I still don't think the great-grandpa had anything sinister to do with Deorr's disappearance. I just think they needed to include him because he was there and after receiving a speeding ticket on July 31st, people may have questioned how his health could be so debilitating that he could not even be considered a person of interest. MOO.