Although age-related deterioration of skeletal muscle strength and size has gained much attention, the age-related increase in adipose tissue within skeletal muscle has not received the same level of attention, especially in older individuals. It is not well known how echo intensity (EI), which is an index of intramuscular adipose tissue (IntraMAT) and connective tissue, is associated with demographic, functional, and morphological characteristics in older individuals. The purpose of this study was to assess the relationships between the EI of the quadriceps femoris (QF) and demographic, functional, and morphological characteristics in pre-old (65-74 years) and old (75 years or older) individuals. One hundred thirty-two men and women aged 65 to 74 years (45 men and 51 women; Pre-old group) or 75 years and older (17 men and 19 women, Old group) participated in this study. The EI and muscle thickness (MT) of the QF were measured using B-mode ultrasonography. Sit-up, supine-up, sit-to-stand, 5-m maximal walk, and 6-min walk tests were performed. The EI of the QF in the Old group (73.1 ± 9.0 a.u.) was significantly higher than that in the Pre-old group (67.6 ± 9.1 a.u.). The EI of the QF was significantly correlated with the MT of the QF in both the Pre-old (r = -0.542, P < 0.01) and Old (r = -0.622, P < 0.01) groups. Stepwise multiple linear regression analysis with the EI of the QF as a dependent variable showed that the MT of the QF was a significant variable in both the Pre-old and Old groups. These results suggest that MT is an extremely valuable factor for explaining the EI of the QF regardless of age group. Exercise interventions are recommended to decrease the risks of metabolic impairment by increasing muscle size and decreasing IntraMAT in individuals aged 65 years or older.