older chicken with persistent watery stool

[BunsenNBeaker]

Hey, I'd love some help I have an 8-year-old chicken who consistently has had diarrhea. I'm constantly having to wash her backside because her vent feathers get so caked. She's free ranged and has clean water and is given fresh pellets multiple times a day along with the other chickens and two turkeys. A few years ago she decided to move into her own Coop away from the rest of the flock by claiming our old rabbit hut which has now been converted into her own personal Coop. She has refused to sleep on a perch for at least 5 years she prefers to sleep inside her Coop; sometimes in the nesting box sometimes just in the shavings. I figure If she's happy I'm happy. Other than diarrhea she seems pretty happy and healthy besides the fact of just getting old. I'm not quite sure what to treat her for; could she have some sort of parasite or something else . How can I rule out one from the other? I'd like to start some course of treatment for her before things get worse, but not quite sure where to start. Any help would be so greatly appreciated, as I'm sure many of you do, I love her dearly and she is a part of the family.
Thank you so much

 

[casportpony]

How's your hen?