Tenn. Code § 8-4015
Any county official as herein defined may after July 1, 1968, retire from office and be paid retirement benefits provided said county official has attained the age of fifty-five (55) years and presents satisfactory evidence of at least twelve (12) years service but less than twenty-four (24) years service, said retirement benefits to be paid as follows:
12 years service - 1/2 of full retirement benefits
13 years service - 13/24 of full retirement benefits
14 years service - 7/12 of full retirement benefits
15 years service - 5/8 of full retirement benefits
16 years service - 2/3 of full retirement benefits
17 years service - 17/24 of full retirement benefits
18 years service - 3/4 of full retirement benefits
19 years service - 19/24 of full retirement benefits
20 years service - 5/6 of full retirement benefits
21 years service - 7/8 of full retirement benefits
22 years service - 11/12 of full retirement benefits
23 years service - 23/24 of full retirement benefits.
T.C.A. § 8-4015