I'd recommend against this, although it is possibly okay at the end of the first chapter.
The reason is quite simple story mechanics. At the beginning of any story, your audience does not feel like they know any of the characters. If they are good, or bad, or villains, or liars, or deceivers. Your audience does not know if they like them. Your characters do not feel like people to them.
That is what first chapters are for, introductions. That is why it is so important for your characters to begin interacting with others in the first 1000 words (about 4 pages of a novel).
A literary agent once told me,
Never open with your character musing about their life, on a run or
sitting on a bus or laying in bed or whatever. Always get to
interaction with another character as fast as possible.
"Musing" is telling, a scene with a conversation and emotion is showing.
But you have to let your audience know about these two characters, and like these two characters, before they give a crap about whether they are in love or not. They don't know what to feel when one stranger tells another stranger "I will love you forever."
The moment falls flat if your audience is watching two strangers vow love. So what?
Readers know they are at the beginning of the story and they have some tolerance for not knowing, but they are expecting character-revealing moments, and this is not really one of them. They have no context for processing what this declaration is supposed to mean, why each character is important to the other.
You'd be better off skipping this declaration and treating it as a past event; open with them already in love, committed, and taking their mutual love for granted. If 13 is adulthood, they can secretly kiss and embrace.
The first chapter is for the "Normal World" of your characters. So make their romantic love an established fact of that normal world; like a married couple they can casually say "love you" on parting; just make this an established relationship. If you need to part them, then when one must depart the Normal World to deal with something, that is a good time to make mutual declarations of undying love -- Not to start a relationship, but to reinforce commitment while they are separated for an unknown amount of time.
It is a common novice-writer mistake to write passages that make perfect sense to them, because they have imagined their characters so much, that when they read what they wrote, all their memories of "who they are" reinforce the text and make it beautiful. But the audience doesn't have all those memories, you have to create them on the page before the same text will make sense to the audience, and seem beautiful.
You have to build up the foundation of the characters first. And that rules out most deep emotional revelations in the very beginning of stories.
Act I is about the first 25% of the story. I think the earliest you can do this is about 3/4 of the way into Act I, after the inciting incident halfway into Act I. So after about 18% of the story, an undying declaration of love might be well received by the audience, they should know enough about the characters, by then, to care about this.
Otherwise, I think it will fall flat, because we (the audience) are observing two strangers we hardly know.