Just a few things to keep in mind, people:
1.) Halfrek posed as Dawn's guidance counselor just into the second half of Season 6. That allows you to place Dawn's age pretty easily.
2.) Being a 97.5% allele match to Emma Frost makes them fraternal twins. Between that, Dawn's age, and the reference to a 'blonde teenager' in the first chapter, you can just as easily place Emma's age.
3.) Jean is said to be a peer of Dawn, albeit slightly older. That establishes a rough age for her age as well.
Please keep those ages in mind when making comments or suggestions about the future of the story or the characters in question.
"Are you really from another universe?"
"Too tired to talk. Come back after I've had my coffee."
"Dawn, you've already had coffee. You're drinking coffee right now. You've got a frappuccino in each hand." Jean sighed, shaking her head in dismay. "How are you still tired? You slept until 7:40, and napped all the way down here. Drooling on my shoulder the whole time, might I add."
Dawn scowled, taking a sip first from the cup in her left hand and then the one in her right. "I hate mornings. Whoever invented times before noon should be shot. And then vamped so they rise and I can shoot them a few more times before staking them. Have I mentioned that I don't like mornings?"
It took Dawn a few steps to realize that she'd lost her traveling companion, and she looked back over her shoulder to find Jean gaping at her in disbelief. Shaking herself, the redhead rushed forward to rejoin Dawn and then leaned in as she lowered her voice. "Vamped… wait, like in vampires? Wherever you're from has vampires?"
"Vampires, werewolves, zombies, demons, witches, magic, the whole nine yards." Dawn took another sip from the cup in her left hand, and then something occurred to her. "You know, this world might have them too. It's just a matter of knowing where to look and, well, I've only been here one night and I spent it at the school." Blessed silence descended as Jean contemplated that, just as Dawn had hoped. Being fully capable of following two-step instructions, Dawn easily led the way as she took alternating sips from her two drinks, guiding Jean westward across Manhattan to the intersection of 7th Avenue and 42nd Street, hanging a left and heading downtown. Half a block later, it was Dawn's turn to stop dead in disbelief. Having spent the last five years in Sunnydale, invoking the phrase 'the strangest thing she'd ever seen' wasn't something she was willing to do lightly. And yet… "What. The fuck. Is that?"
'That' was a man being chased up the street by a figure in red and blue spandex that bounced from streetlight to streetlight, occasionally firing off some sort of line that allowed him to swing forward past his prey, heckling the man as he went. Jean's loud sigh drew Dawn's attention to the redhead as she shook her head in disgust. "That's Spider-Man. Him and his girlfriend are pretty much walking public relations nightmares for our kind, and they're not even mutants according to the professor. But since they use what are obviously superhuman powers to fight crime…"
Over the years, Dawn had gotten enough 'your conduct reflects on me' lectures from both of her parents plus Buffy to be able to finish the redhead's thought for her. "People assume they're mutants, and you get blamed for what he does. That sucks." Something else that sucked? The fact that Spider-Man was creating his spectacle between them and their destination, when they had only thirty minutes to make what her phone said was a twenty-one minute walk. After watching the superhero play with his prey for a few more seconds, Dawn turned to Jean and thrust her frappuccinos into the telepath's hands. "Well, as long as we're getting blamed anyway…"
"Dawn, what are you… oh no, please don't…"
Dawn ignored the redhead's plaintive cries as she stalked toward Spider-Man and a scruffy-looking man she assumed was a criminal of some sort. "Hey! Crooky McCrookerson!" The crook skidded to a stop and looked around wildly before gesturing to himself. "Yes, you. I have a very important question to ask you." Stopping an arm's length away from the man, Dawn raised her right hand high and wiggled her fingers. "What did the five fingers say to the face?"
With a thought, Dawn's hand shifted into glittering diamond and she smacked the man across the face hard. "Slap!" The hit spun the man clear around before dumping him onto the ground, leaving him rolling around on the sidewalk as he clutched at his jaw and moaned. Oh, and… "Are you… are you crying? There's no crying in burglary." Taking a step forward as her hand returned to normal, Dawn stretched out a toe and kicked an oversized, bright pink purse away from the fallen crook. "At least I hope you stole this, because even I know it doesn't go with your outfit."
Spider-Man picked that moment to remind her of his presence, firing one of his weird white lines down and retrieving the purse from the sidewalk. "I'm not sure it goes with anyone's outfit, to be honest. Spider-Woman bought it because it looked like the kind of thing you'd own if you had more money than taste or common sense." Dawn looked up at him, raising an eyebrow. "It's a bait purse. She dresses up like a ditzy cheerleader on vacation and wanders around until someone robs her. There's a hidden pocket in the lining where we hide an iPhone, so we can track the crook down if they get away."
"Ah." Tilting her head to the side, Dawn ran a critical eye over the superhero. "So… you're Spider-Man. Are there a lot of red and blue spiders out there in the wild?" Behind her, she could hear Jean snicker as she approached, even as Spider-Man sputtered. "Let me guess. You got it for ninety percent off in a 'nobody else wants to wear this' sale?"
Grumbling under his breath, Spider-Man tipped forward so he was hanging from the underside of the streetlight, clutching the reclaimed purse to his chest. "I'll have you know that I put a lot of time and effort into designing this costume, thank you very much. I wanted something that I could wear after dark without terrifying the crap out of the people I was saving."
Dawn scoffed at that. "You know what'd go a long way there? People being able to see your face." Before she could share any additional thoughts on Spider-Man's choice of costume, or the little entrapment-esque operation he was running with Spider-Woman, the sound of sirens cut through the ambient noise of downtown Manhattan. "Well, it's been fun, but I do have a train to catch." Looking back over her shoulder, she met Jean's gaze and then jerked her head in the direction of the distant Penn Station. "Hey, Red! Unless you want to be stuck listening to me whine for however long it takes to sort shit out and get us on a later train… let's move it!"
"Woah woah woah." Doing a flip in midair as he dropped to the street, Spider-Man landed neatly on his feet and straightened up before gesturing to the downed criminal. "You can't leave. I'm pretty sure you bitchslapped that man's jaw into at least two pieces. Possibly more."
"And you're going to be the one still here when the cops arrive. Have fun with that." Taking a step to the left, Dawn attempted to slip past Spider-Man only to sigh as he cut her off again. "All right, listen. I'm not a morning person and I've only had one coffee so far today. I'm pretty sure you've made me late for the train that I'm supposed to be taking to my first ever meeting with my biological family. And my fists aren't the only part of me I can turn into diamond." She shot a pointed look at his crotch before slowly bringing her eyes back up to stare into the reflective white lenses of his mask. "Your move, bug boy."
A strange combination of thump and metallic twang drew Dawn's attention upward to where a pigtailed blonde girl wearing an Aéropostale t-shirt, distressed jeans, and a black domino mask was perched on the streetlight that Spider-Man had previously occupied. Spider-Woman, presumably. "If it means anything, I'd like to cast a vote for the option that doesn't end with him peeing blood. I'd probably end up getting graphic details of the damage from his girlfriend and…" Trailing off, the girl shuddered. "No thanks."
Spider-Man looked from Dawn to his partner and back before grimacing and taking two big steps backward. "Uh, now that I think about it, you have a very valid point. Very convincing. There's no need to stick around. As a matter of fact… do you want a lift to the station?"