No it's not, unless hyperventilating on pure oxygen (it's ridiculous that this is allowed in competition). I used to be a medical instructor for AIDA (now retired). Anything above 5 minutes is not simply a matter of training.

One can put years into apnea walks and breath tables and never approach six minutes. Genetics (lung volume) and the body's ability to cope with increased CO2 levels are the critical factors.

You are right. I've looked up the apnoe training tables again, and the limit is 5 min indeed.

from our research, 6 minutes is actually hard, but not impossible. We took 40 seconds, because its an average a normal person can do. If it's something below, you need to be aware of doing some exercise or changing your smoking habits(if any) or consult a doctor.