No, they don't.https://www.google.com/search?q=average+age+boys+stop+growing&oq=average&aqs=chrome.0.69i59j69i57j35i39j0i512l2j0i433i512j0i433i457i512j0i402j0i271l3.1317j0j4&client=ms-android-oneplus-rvo3&sourceid=chrome-mobile&ie=UTF-8#ip=1You can easily see differently by simply googling this.Most resources say 16. Some say 16-18.Most boys do not continue to grow past 19. Some do. Most do not.This is why male growth charts norms plateau at 16-17 years, but weight continues upward.It might be commonly said to be 19-21 in your country's education system, but that's not accurate.If you're able to find a source that says it's typical to keep growing into your 20s, there 100 others that say it's not.This is common pediatric knowledge.You think you're right here, because lay people have told you this while growing up, but it's not medically accurate.
It is just quite frankly wrong, every countries education system and reliable sources all say boys stop growing at about 19-21