Flexibility is one of the basic physical qualities that are worked on both in elementary and secondary education but not at the university stage. Of the four basic physical qualities, flexibility is the only quality that progressively diminishes with age. We selected a sample of 410 subjects (42% males vs. 58% females) aged 6 to 30 years old from Granada and Almería. To collect data, the deep trunk flexion test was used, establishing that the mean flexibility among the sample was 22.25 cm. By gender, females (X = 22.82 cm) were more flexible than males (X = 21.46 cm). The highest flexor capacity values were achieved between the ages of 10 and 11 years, with these values steadily decreasing with age. Consequently, the highest peaks of flexibility were found in primary education (24.98 cm) while the lowest values were found in university education (19.50 cm). Finally, as reported in the literature consulted, the results of this study found that average flexibility decreases with age.