Dear Mr./Ms. Bossname
It is with great regret that I offer my letter of resignation from (company), effective (date).
I have only ever had positive experiences at this company, and I am sorry to be leaving. However, my advancing age and declining health mean that I can no longer adequately perform my duties as a (position). I will take this time to see to my health needs and spend time with my family.
Thank you for the opportunities you and this company have offered me, and thank you in advance for your understanding regarding my situation.