Background: This study aimed to describe trends over time regarding disparities in treatment and relative survival (RS) between younger and older patients with non-small cell lung cancer (NSCLC). Methods: All patients diagnosed with pathologically verified NSCLC in 1990-2014 were included from the Netherlands Cancer Registry (n = 187,315). Treatment and RS (adjusted for sex, histology and treatment) were analyzed according to age group ( <70 years versus >= 70 years), stage and five-year period of diagnosis. Results: Between 1990 and 2014, five-year RS increased from 17 to 22% among younger patients and from 12 to 16% among elderly. The application of surgery increased over time for elderly with stage I NSCLC, decreased for elderly with stage II, and was stable but higher for younger patients. Disparities in RS between age groups with stage I became smaller since 2000-2004, but did not change over time for stage II. For stage III and IV, both age groups showed strong increases over time in chemoradiotherapy and chemotherapy from 2000 onwards, although considerably less among elderly. One-, three- and five-year RS increased more strongly over time for the younger group leading to larger disparities between age groups with stage III or IV NSCLC. Conclusion: More curative-intent treatment and improved RS for NSCLC were seen over time, but were less profound among elderly. Disparities herein between age groups seemed to become smaller over time for stage I NSCLC, did not change for stage II, and were widening for stage III and IV at the expense of elderly. Future prospective studies should focus on optimizing treatment selection and outcomes for elderly.