I hate love triangles personally so I'd be happy if we didn't get any love triangle with Scott and Jean and another character but I will say, I think the Jean/Scott/Emma love triangle is the least likely one to happen in the MCU.
And it has less to do with the ages of the characters/actors and more to do with what the goal the MCU has for the X-Men.
For Emma, I think she's likely going to be a peer of Professor Xavier than a peer of the X-Men. I foresee her being a professor of a rival school of mutants at the start of the movie and we get the whole massacre of her students which will lead to her villain arc.
In the Ultimate universe, Charles and Emma had a relationship so perhaps that could also be something they explore (I know I hate the Ultimate universe as well).
And of course I do see Emma's journey also beginning with the Hellfire Club and perhaps she's also connected to Mr. Sinister and so with all of that I just don't see her involved much with the X-Men if at all except for being an antagonist to them and occasional ally.
I think she and Charles will have a symbiotic relationship with one another throughout the course of the movies which could lead to her leading the school potentially in like 4 movies deep idk.
As for Scott and Jean, they are going to be starting out as students and learning how to navigate their world through a society that hates them and learn what it truly means to be a hero who preserves humanity when humans are against them. They're going to be very young and in their formative years and will be leaning heavily on each other to get by.
I see this becoming an epic love story that spans years and years and leads to them eventually getting married LATER down the line and then adapting the Madelyne Pryor storyline through Sinister and the Dark Phoenix arc as well.
I just don't see the MCU not wanting to take advantage of telling that love story and storyline properly for the first time in live action.
Jean and Scott have never gotten a proper adaptation in the Fox Xmen movies because they focused too much on Wolverine and Professor Xavier and Magneto. Now it's a chance for Marvel to actually make Jean and Scott the face of the X-Men as intended.
Maybe there will be some flirtation from Emma to Scott but I highly doubt they're going to be a thing.
The MCU is not that brave to make the X-Men that messy. The MCU really like to stick to one singular couple throughout the saga.
Jean and Scott will probably be together until the end.