Here are the official rules by the WTF (World Tae Kwon Do Federation), regarding Poom and Dan certification.

Students under the age of 15 years old will receive what is called a “Poom Certificate” when they achieve 1st Degree Black Belt. Students above the age of 15 years old will receive a “Dan Certificate” when they achieve 1st Degree Black Belt.

Here is how it works: (see chart below)

Under 15 years old: A student can achieve 1st Poom, then 2nd Poom, then 3rd Poom under the age of 15 years old. Then, when the child/student turns 15 years old, they can simply convert the Poom to the equal rank “Dan Certificate”. There may be a small fee to do so – please check with your location’s director.

So for example, a 12 year old, 2nd Poom, Black Belt will be a 2nd Degree (Dan) Black Belt when he or she turns 15 years old.

If a child achieves 4th Poom Black Belt, then that student will need to wait till they turn 18 years old to convert that 4th Poom to a 4th Degree (Dan) Certificate.

Note that the WTF may award Dans outside these rules in exceptional situations, for example rewarding national champions in Olympic TKD Sparring.