(First of, This might not exactly solve much for this continuity problem But I thought this might finally resolve from events of the timeline from the recent episodes)
So ever since the episode of the curse of the Blood Moon, Tom revealed that it has been two years since the event has happened. and from that, fans are in mild confusion to believe that the age and timeline could really be off. Meaning that Star should actually be 16 years old instead of 15.
For to at least attempt to clear this out, she is really 15 still. Although what Tom said was the next blood moon is in 665 years away. He did Not say what day it's in. Now I know where everyone is confused with, "If two years have passed would Star be 16 by now?" Well, I think we can actually clear this timeline thanks to one of the recent episodes.
If you remember from Ghost of Butterfly Castle, the scene when the villager revealed about who birthday was, you'll see that the date for it on the planner book was set in March. Which could mean now they could be in that mouth. (Which made me think the Blood Moon epi should've been a valentines epi) So the 2nd year is just beginning
Then came across "What about Stump day?" now to describe that may have been set before the event of Skooled. Since it's a holiday special placed in randomly in time for Christmas. It makes sense since it doesn't even connect with the previous episode Monster Bash. It can be placed anywhere before the Season 3 finale and it possibly wouldn't make a difference
Hopefully, this might clear this out. What do you all really think?
I agree with you, it can be two years but understand that Star's birthday is on christmas essentially, assuming the calendar between Earth and Mewni is similar, so she doesn't turn until the end, Marco's birthday is only about a month or so before hers.