Background/Objective <p>Overweight and obesity are associated with insulin resistance. However, the importance of the time of onset of excess body fat is unknown and was presently examined.</p> Subjects/Methods <p>We included 339 adult participants having information about their body mass index (BMI) at 18 years of age. Insulin action was determined as homeostasis model assessment (HOMA-IR) reflecting liver, hyperinsulinemic euglycemic clamp reflecting skeletal muscle, Adipo-IR reflecting adipose tissue in vivo, and insulin action on lipogeneses reflecting fat cells. The subjects were divided into never having overweight/obesity with BMI always &lt;25 kg/m² (NO), having BMI ≥ 25 kg/m² already at 18 years of age (EO), and late onset of overweight when BMI was ≥25 kg/m² only at current examination (LO). The groups were compared by unpaired <i>t</i>-test and single and multiple regression analysis (the latter to study the influence of other factors than insulin action).</p> Results <p>EO had 5 kg/m² higher BMI and was 10 years younger than LO at examination (<i>p</i> &lt; 0.0001). EO was more insulin resistant than LO for both HOMA-IR and Adipo-IR, but not clamp (<i>p</i> = 0.01, 0.02, and 0.11, respectively). However, when the different measures of insulin resistance were corrected for current BMI or age there were no significant differences between EO and LO for any of the measures of insulin action (<i>p</i> ≥ 0.08). Furthermore, in all subjects current BMI (<i>p</i> &lt; 0.0001) but not BMI when 18 years old (<i>p</i> ≥ 0.13) correlated with different insulin resistance measures.</p> Conclusion <p>When current BMI or age is considered, there is no difference between early or late onset of overweight/obesity for the level of insulin resistance in the different target tissues of the hormone.</p>