In general, younger children have difficulty holding arnis equipment and learning the patterns. It is difficult for them to train safely. While not a Filipino requirement, the Soon clan also has a general prohibition against teaching martial systems to anyone below the age of 12. The Japanese color ranking system has been imported into Arnis, although it is not traditional to have such ranks. You may train with rank or without.
First degree is relatively quick.
It can be done with a steady, moderate training regimen in 2-3 years. Remember, Arnis is pattern based. In hand to hand, you must be able to generate power to be effective, and your body must be able to tolerate generating power. This alone bumps up the training curve. Arnis is more fault-tolerant because of the weapons, and so first degree is acquired when a reasonable command of the patterns and rhythms are acquired.