This is my interpretation, based on context and drawing conclusions:
I think we can track the ages thanks to Lloyd, of all people; like you said, in Rise of the Snakes Lloyd is thought to be around 8 years old, and once he got hit by the tea in Legacy of the Green Ninja, he was aged up to the Ninja’s ages. Since Wu at least trusts them to babysit Lloyd, I’d expect them to be about 13 years old (an age many of my friends, myself included, were allowed to begin babysitting), and Nya was 12 since she’s younger than Kai. Lloyd was physically aged up five years while still being 8 years old.
Flash forward to Sons of Garmadon, which is, our time, six years from LotGN. Assuming that the Ninjago timeline at least loosely follows our time, that would mean Lloyd is around 14 or 15 while biologically 18, while Kai, Jay and Cole are 18-19 and Nya is 17. This means that in SoG, Lloyd would have been older than the Ninja were when they started. Harumi would be 15, which allows for the ‘tall-legs’ since she could have hit her growth spurt.
Furthermore, working under the presumption that March of the Oni takes place very soon after SoG and Hunted, that would mean that Jay and Nya would be around 18 and 17 respectively, which works for the Yin and Yang proposal since they’re too young for marriage but old enough to be thinking about it.
But those are just what I think, based on context clues and a healthy dose of assumption. But, I think it works well enough.