We know from Harumi's speech in Sons of Garmadon that Lloyd is the youngest ninja. Nya is known to be younger than Kai. I see Jay as somewhere between Kai and Nya in age, and I have always viewed Cole as the oldest biological ninja (Ex. he was the original leader, and adopted something of a "big brother" role in early seasons). I assume that Zane was built around the same time as the Serpentine Wars (correct me if I'm wrong).
Because of this, my current and personal age-ranking (from oldest to youngest) is as follows:
I came up with these ages through consideration of my intuitions of the character's initial ages paired with my interpretation of the seasonal timeline.
In Rise of the Snakes, which takes place soon after the Pilots, the ninja are in their earlier years. Lloyd is 8-10 years old during this time, so his age doesn't affect the ranking yet. Nya's behavior and mannerisms lead me to place her at 14, which means that Jay is 15, Kai is 16, and Cole is 17 for the first season.
In Legacy of the Green Ninja, not much time has passed, and the Ninja's ages remain the same-- aside from Lloyd, who ages up to be a teenager. While it's clear to me that he's a teenager like the other ninja, he is definitely the youngest, which leads me to place him at 13 years old for this season.
Ninjago undergoes so much technological advancement between Legacy and Rebooted that, realistically, decades should have passed-- but evidently this is not the case, and for the purposes of maintaining in-show lore, I assume that about a year has passed. This means that Lloyd is 14, Nya is 15, Jay is 16, Kai is 17, and Cole is 18 during Rebooted.
Between Rebooted and Tournament of Elements, the ninja have had time to mourn Zane and pursue separate lives-- Jay starts up a successful TV show, etc. At least another year has passed to facilitate this-- meaning that for Tournament, Lloyd is 15, Nya is 16, Jay is 17, Kai is 18, and Cole is 19.
Possession, Skybound, Day of the Departed, and Hands of Time seem to occur in rapid succession (with most of Skybound removed from the timeline), and I group these seasons within the same year as Tournament-- meaning that no one ages up in the meantime.
We are told that one year has passed between Hands of Time and Sons of Garmadon (thank you, Cole!), meaning that the ages change again-- Lloyd is 16, Nya is 17, Jay is 18, Kai is 19, and Cole is 20. Seasons Hunted through Prime Empire occur (I think) fairly successively, which leaves me with the current age-ranking listed above.
I am satisfied with this list, because I think it reflects the character progression of the ninja with decent accuracy. It strikes me as unrealistic to place Lloyd as any younger than 16 after the design change (starting with Sons of Garmadon,) and the fact that his new design is based on that of his movie counterpart (who is 16 in canon) validates it for me. While Cole being 20 contradicts the in-show statement that they are all still teenagers as of Hunted, I think it is still close enough to count, and I do not mind that he is the sole outlier. If I need to, I could make Kai and Jay the same age and bring Cole down a year. An obvious flaw in my hypothesis is the fact that a few more in-story years will age most of the Ninja out of their teens, but this problem will come up eventually regardless of which ages you prefer.
This list is entirely my own opinion. Does anyone have suggestions as to its improvement?