Judo has rank requirements that include competition record, skills demonstration, time in rank, minimum age, service, etc. The specific requirements are dictated by a judoka's governing organization, and these requirements will not necessarily match, which is why a judoka's recognized rank may differ depending on the considering organization. For example, Keiko Fukuda was recognized as a 10th dan (judan) by United States organizations, but only as a 9th dan (kudan) by the mother Kodokan organization.
I am most familiar with the United States Judo Federation requirements. For the black belt ranks that a 21 year old could attain, the minimum ages are:
- Shodan 14 yrs
- Nidan 16 yrs
- Sandan 18 yrs
- Yondan 21 yrs
For these ranks, the most important factor would be competition record. World/Olympic medalists could reach 4th dan (yondan) by age 21, but this would be quite exceptional and require you to be known worldwide. Receiving a shodan at age 14 is also quite unusual because adult/senior ranks are usually granted starting at age 17; you would have to be able to consistently defeat adults while still developing physically, or more recently, completely dominate junior divisions at national tournaments.
Here are my rough expectations for ranks at age 21:
- 1st (Shodan) minimum expected for continuous practice since 4/5 until 21
- 2nd (Nidan) accomplished competitor, not necessarily known outside region
- 3rd (Sandan) National Champion
- 4th (Yondan) World/Olympic champion
Also be aware that some judoka are unconcerned with their rank. There are shodans who are technically and competitively better than upper ranks, but they simply do not bother with further promotion paperwork upon receiving a black belt.