I found a post recently discussing how old the ninjas are exactly and decided to do my own take on it (and I needed it for the fan fic I'm writing) I initially wanted to give each season a year, but after thinking it over and realising how old they would be then, I kind of changed my mind.
I'd put the pilot episodes a year before season 1, and then I would put seasons 1&2 in one year because they seem to be happening one after another.
I would place rebooted happening a year after the defeat of Overlord (nr 1) in winter, then the Tournament taking place somewhere in late summer/early autumn, enough time had to pass for the ninjas to give up on Zane and go their separate ways. Then Possession right after it, going late autumn/early winter of that year.
Skybound would be in late spring/early summer of next year, going on the fact that Nadakhan mentioned something about "ghosts are so last year", but I wouldn't bet on it. Then Hands of time within that same year/time caused by that whole time travel thing.
Then the Oni seasons (8-10) spamming about the whole of next year, because I feel like Lloyd and the others have been in hiding for some time already by the time Haunted is happening, but again, I wouldn't bet on it, and it's just a feeling I have.
Then Season 11 (parts 1&2) is happening, again, next year after Ninjago is rebuilt. I would say that the Fire Chapter takes about a month, and then the Ice Chapter doesn't affect the Ninjago Time cause of the time difference, but I feel like it would be around a month also, two max.
I think Prime Empire and Master of the Mountain take place in the same year, together spanning about two months, maybe three if the travel to Shintaro takes a while, but I'm not sure about that. I also think it's not that long after the Oni seasons, like just a few months apart, despite being in different years on the calendar, considering how Lloyd reacts and acts about the "princesses can't be trusted" meaning Harumi must still be rather fresh in his mind for him to act like that.
Then there's Seabound, which I think takes place at the beginning of the next year, and then we have a time skip of about a year, considering how all the ninjas had other lives and jobs set up and the state Jay's in. I think it's a similar situation to what happened after Rebooted.
And at the end, we have Crystalised, happening shortly after Nya is returned to her physical body.
Also, I'm thinking the Merge happens about a year after that, with about another 3-2 years before the events of Dragons Rising Season 1 take place (I'm thinking about the fact that Arin is a full-on teenager as opposed to the child he is when Lloyd talks to him during the Merge)
So, summing up - between the pilot episodes and Crystalised, there would be an 8-year difference, if my math is correct.
Now onto the real reason this discussion even began, the ninjas' ages!
I'd put the OG 4 at about the same age (with Zane being counted as his mental age rather than chronological), and I'd say they are 14-15 during the pilot episodes.
Serpentine & Legacy of the Green Ninja: - 16-17 for OG 4 - Nya (as she is two years younger than Kai) would be 14-15. - I put Lloyd before Tomorrow's Tea at 9-10 and after at 13.
Rebooted: - OG 4: 17-18 - Nya: 15-16 - Lloyd: 14
Tournament of Elements & Possession: - OG 4: 18-19 - Nya: 16-17 - Lloyd: 15
Skybound/Hands of Time: - OG 4: 19-20 - Nya: 17-18 (I'm thinking it makes sense because while I have no doubt Nadakhan would have no reservations about Delara's age, I think a truly young teenage girl would not willingly end up in such a situation and as we saw when he summoned her ghost to posses Nya, she seemed quite willing and in love. But keep in mind that I have absolutely no idea how romantic love works, so yeah, and Nya looks too much like her to be too different in age. Also, the way Delara speaks just makes her sound older, or maybe it's just the time she's from?) - Lloyd: 16
- This would also work as the ninjas were put in prison here. And they would, technically, need to be adults for that. Lloyd being 16 also allows, I think, for the law to treat him as an adult (at least that's how it somewhat works in my country, from what I know, I'm no lawyer)
Oni Seasons: - OG 4: 20-21 - Nya: 18-19 - Lloyd: 17 (his voice starts to crack and all that.)
The Forbidden Spinjitzu, Prime Empire & Master of The Mountain: - OG 4: 21-22 - Nya: 19-20 - Lloyd: 18
Seabound & aftermatch: - OG 4: 22-23 - Nya: 20-21 - Lloyd: 19
Crystalised: - OG 4: 23-24 - Nya: 21-22 - Lloyd: 20
Merge: - OG 4: 24-25 - Nya: 22-23 - Lloyd: 21
Dragons Rising Season 1&2(?): - OG 4: 26-28 - Nya: 24-26 - Lloyd: 23-25
I think this makes the most sense to me, which everyone ending up at the ages they do in DR makes it more believable that they manage to live on and all that. Also, Lloyd being an actual master in it makes more sense if he is in his 20s and has been training for around a decade. So, once again, between the pilot episodes and the DR, there are at least 9 years and 11 at most. If my estimations are correct, that is.