We were in a somewhat comparable situation this year. DS8, in third grade, could have skipped directly into fifth grade in a congregated gifted program - heavily STEM focused, too.

He decided he didn't want to, which was the crucial factor - however other factors came into play for me to decide not to override DS wishes (I do that all the time, he is often too anxious to pick what I know will be the right thing for him):

He is really weak in sports - so weak I'd be seriously worried about his somewhat tenuous social confidence if he were any weaker in comparison to his classmates. He was entered early with a birthday weeks after the cutoff, but as there is no red shirting in that school he's the youngest only by a few weeks up to a year - and STILL struggles to get Cs. I shudder to think how he'd struggle at the age of 8, in fifth grade, in a new school, possibly with red shirted boys in his class. He is tall and thin, moves a lot, swims, bikes, does martial arts, but is very very uncoordinated and physically anxious. Do grades in sports count? They did to me when I was a kid, and to everyone else.
So while I m not worried about actually competing in athletics, I m worried about competing socially, as it were. And I think he might really care about being competitive for academic competitions, or in band, or orchestra, or fine arts. (Which may affect his college options where we live, though athletics won't.) And while he is advanced in LA as well, he's just not as "out there" as he is in science and math (where he's been coasting along happily in fourth grade this year already anyway. Don't know what well do next year when he is in fourth and would have to go to middle school for math but we'll cross that bridge when we come to it).
And he has complained about being the youngest in class already, at the same time as complaining about the rest of the class being SO slow.
So like you, I have hopes that fifth grade in the gifted STEM program might finally, finally be able to offer him that sweet spot of being with age peers (they usually have a number of skipped kids and the occasional double skipped kid as well as grade level kids so he'll be smack bang in the middle) AND being academically appropriately placed.
As I said, athletics won't affect his college options, the most important considerations for us were social vs academic fit now.
I wouldn't plan for what could happen when she's 17.