Wisdom teeth do not have to be removed at age 21.
Generally speaking, if the wisdom teeth are growing in a normal direction and are involved in chewing, they do not need to be removed. Secondly, if the tooth is in the alveolar bone and is no longer developing, it does not need to be removed.
If there is little room for wisdom teeth to grow and they often induce pericoronitis, then you may consider removing them in a timely manner. In addition if the wisdom tooth grows in the wrong direction and becomes anteriorly tilted, it may press on the second molar, which in turn may cause food inlays. If the problem recurs, it may cause periodontitis, caries, loose teeth and so on. In such cases, prophylactic removal of wisdom teeth is likewise recommended.
Whether or not wisdom teeth need to be removed should be determined on a case-by-case basis. When wisdom teeth cause a lot of inconvenience in your life, you may consider removing them as early as possible to avoid affecting your oral health.