The mean levels of percentage body fat at age 8 were 28% for boys and 31% for girls, and at age 19 were 23% for boys and 35% for girls. Table 2 contains the smoothed estimates of percentage body fat for every fifth percentile from the 5th to 95th percentiles for the sex-specific 6-month age groups.Figures 2 and 3 show the smoothed estimates for the 15th, 50th, 85th, and 95th percentiles, for boys and girls, respectively.elected percentiles of smoothed percentage body fat among boys aged 8–19 years: United States, 1999–2004 Percentage body fat SOURCE: CDC/NCHS, National Health and Nutrition Examination Survey, 1999–2004.