Abstract Background ST-segment myocardial infarction (STEMI) is typically associated with increased age, but there is an important group of patients who suffer from STEMI under the age of 50 who are not well characterized in studies. Methods and results We analysed results from Myocardial Ischemia National Audit Project (MINAP) from the United Kingdom (UK) between 2010 and 2017 and the National Inpatient Sample (NIS) from the United States of America (USA) between 2010 and 2018. After exclusion criteria, there were 32 719 STEMI patients aged ≤50 from MINAP, and 238 952 patients’ ≤50 from the NIS. We analysed temporal trends in demographics, management, and mortality. The proportion of females increased, 15.6% (2010–2012) to 17.6% (2016–2017) (UK) and 22.8% (2010–2012) to 23.1% (2016–2018) (USA). The proportion of white patients decreased, from 86.7% (2010) to 79.1% (2017) (UK) and 72.1% (2010) to 67.1% (2017) (USA). Invasive coronary angiography (ICA) rates increased in UK (2010–2012: 89.0%, 2016–2017: 94.3%), while decreased in USA (2010–2012: 88.9%, 2016–2018: 86.2% (USA). After adjusting for baseline characteristics and management strategies, there was no difference in all-cause mortality in the UK in 2016–2017 compared to 2010–2012 (OR:1.21, 95% CI:0.60–2.40), but there was a decrease in the USA in 2016–2018 compared to 2010–2012 (OR: 0.84, 95% CI: 0.79–0.90). Conclusion The demographics of young STEMI patients have temporally changed in the UK and USA, with increased proportions of females and ethnic minorities. There was a significant increase in the frequency of diabetes mellitus over the respective time periods in both countries.