Board Thread:Theories and Rumors/@comment-5926733-20130609193523/@comment-26831292-20150729033230

"Young age" to Garmadon could mean several hundred years, since he ages a lot slower. So part of the reason may have been that Garmadon was already 400 years old when he a married a 20- year old Misako.

The other part is that Garmadon and Misako share a blood relationship, so some of the genes Lloyd had inherited from his father, Misako may have also gained, although she won't live nearly as long (Possibly 600 years) as Garmadon's life expectancy.

But, you do have a point. The Ninjago storyline contains so many inconsistencies, it is nearly impossble to create an explanation to accommodate for all of its flaws.