There is a paper applying it to very old people, who are known to be affected by cognitive decline: 

Kivett, V.R., Stevenson, M.L. and Zwane, C.H., 2000. Very-old rural adults: Functional status and social support. Journal of Applied Gerontology, 19(1), pp.58-77.

Hello Noud

I have not accessed the full text of this, but in the Google summary, Cantril is mentioned:

Wellman, J., Barter, B., & Steele, A. (2015). Group top-up therapy for service users: Joshua Wellman and colleagues argue that those with learning disabilities could benefit from having a mix of individual and group therapy. Learning Disability Practice, 18(8), 33-38.

https://www.researchgate.net/publication/282329285_Group_top-up_therapy_for_service_users

There is one reference to Cantril  in section 4.2.1 of this thesis:

Martindale, K. J. (2010). An investigation into the subjective wellbeing of people with an intellectual disability (No. Ph. D.). Deakin University.

http://vhost47.hosted-sites.deakin.edu.au/publications/resources/thesis-martindale-k.pdf

Also see this RG link:

https://www.researchgate.net/publication/228722481_An_investigation_into_the_subjective_wellbeing_of_people_with_an_intellectual_disability

This might be relevant:

Riemsma, R. P., Forbes, C. A., Glanville, J. M., Eastwood, A. J., & Kleijnen, J. (2000). General health status measures for people with cognitive impairment: learning disability and acquired brain injury. Health technology assessment (Winchester, England), 5(6), 1-100.

This is the full text:

http://www.journalslibrary.nihr.ac.uk/__data/assets/pdf_file/0003/64965/FullReport-hta5060.pdf

This is the RG link:

https://www.researchgate.net/publication/12016093_General_health_status_measures_for_people_with_cognitive_impairment_Learning_disability_and_acquired_brain_injury

And this is one of the references:

Ormel, J., Kempen, G. I., Deeg, D. J., Brilman, E. I., Sonderen, E., & Relyveld, J. (1998). Functioning, Well‐Being, and Health Perception in Late Middle‐Aged and Older People: Comparing the Effects of Depressive Symptoms and Chronic Medical Conditions. Journal of the American Geriatrics Society, 46(1), 39-48.

https://www.researchgate.net/publication/13795147

I hope there is something of help here.

Very best wishes for your PhD,

Mary

Article General health status measures for people with cognitive imp...

Article Functioning, Well-Being, and Health Perception in Late Middl...

Article An investigation into the subjective wellbeing of people wit...

Article Group top-up therapy for service users

Dear Dragos and Mary, thank you very much for your suggestions!

Noud