This takes place on Stump Day Eve. Marco thought it would be a better idea to celebrate Star's birthday the day before her real birthday, rather than on it.

"Happy birthday to you! Happy birthday to you! Happy birthday dear Star! Happy birthday to you!"

Star sucked in a deep breath and extinguished eighteen candles in quick succession. The Diaz's applauded and Marco turned the lights back on before returning to his seat at his girlfriend's side.

Raphael picked up the cake. "I will cut the cake. Be right back!" And he headed for the kitchen.

"Cake! Cake!" three-year-old Mariposa cheered, pursuing her father into the kitchen as if the cake would disappear if it left her line of sight.

Star turned to look at Marco beside her and returned his warm smile. "This is perfect. Thank you, Marco."

"You're welcome, Star." He gave her a peck on the cheek. "We'll throw you a big party with all our friends and family after New Year's, but I at least wanted to celebrate with you close to your birthday."

"Awww…" Star's blue eyes sparkled as she gazed lovingly at her boyfriend. "Marco you are just so sweet-"

"So, Star," Angie said, interrupting the teens' conversation before it could become any more saccharin. "How's it feel to officially be an adult?"

Star stared blankly. "Say what now?"

"On Earth, when you turn eighteen years old, you're legally considered an adult," Marco explained. "Well, in most of the world. As far as I know, Earthni law has remained mostly the same on that subject."

Star blinked. "…Adult?"

Rafael returned, carrying plates baring slices of cake, and followed by Mariposa still chanting "Cake! Cake! Cake!" Star's slice was approximately one entire quarter of the whole confection. Briefly forgetting about Mrs. Diaz's question, she eagerly dug into her helping of dessert. Mr. Diaz was just as much an artist in the kitchen as he was with a paint brush, and she lost herself in the sugar.

But unfortunately, Mrs. Diaz's inquiry would not be entirely forgotten…

Marco was thirsty. He glanced at the clock: 2:16am. Too long to wait until morning. He swung his legs out of bed and headed for the stairs. Sure, he could just drink some tap water out of the bathroom sink, but he wanted ice. Room temperature water just didn't have the same refreshing properties on a parched throat.

Descending the stairs into the living room, he was surprised to find the TV on with sound turned off. Star was lying on her back on the couch, paying no attention to the screen.

He approached slowly. "Star?"

She barely glanced at him. "Oh. Hey, Marco."

"What are you doing up? Shouldn't you be asleep with visions of tree stumps dancing in your head or…whatever it is you're supposed to dream about on Stump Day Eve?"

She shrugged. "Probably."

Marco was a bit perplexed, and not entirely sure how to respond. It definitely was not like Star to be this unenthusiastic about her favorite holiday. He glanced at the TV. "You know the sound is off, right?"

"Mhm. I'm not watching anything. I just put it on for company."

Marco moved to sit on the couch, and Star sat up to give him space to do so. "You okay?"

"I'm fine."

"But something's on your mind. Otherwise you wouldn't be up at 2 in the morning."

Star sighed. "I was just…thinking about what your mom said. 'How's it feel to be an adult?'"

"Okay…?"

"Being an adult just sounds so…stupid and dumb and boring!"

Marco chuckled. "Be glad you're not a princess anymore."

Star raised an eyebrow. "Why?"

"When I was living with you and your parents at the castle, when you weren't around I spent a lot of time in the Royal library."

"…Why?"

Marco shrugged. "I like learning about stuff. When the princess turns 18, she begins intensive training to become queen. Like, practically 24/7. Her location must be monitored by the Royal guards at all times, and she's barely allowed to leave the castle. She has to dress a certain way, eat specific things, and she definitely isn't allowed to go exploring the multiverse."

As Marco listed off rules, Star's expression became progressively more disgusted. "That sounds horrible!"

"Sure does."

Star slumped down and crossed her arms. "Well, gee, when you put it like that, I guess being an adult could be way worse. I could be an adult and be a princess. Woof!"

Marco put his arm around Star's shoulders and pulled her close. She nestled her head into his side. "I know being an adult sounds boring. Responsibilities galore, paying bills, having a job and no free time…"

Star just groaned and tried to bury her head further into Marco's side.

"But I think you're already more adult than you realize."

She looked up at him quizzically. "How do you mean?"

"For one thing, most fifteen-year-olds don't do battle against giant, soul-sucking monsters. Actually, come to think of it, most adults don't do that, either."

This elicited a small giggle from his girlfriend.

"You worked so hard to help Eclipsa bring both sides of the kingdom together, and you've kept working hard for the last three years to help the people of Earth and Mewmans and monsters learn to coexist. That's not an easy task for anyone to undertake."

That was no exaggeration. "It's so harrrrrrrd."

"And how many times did my parents leave you totally alone with Mariposa when she was an infant? I can't imagine they'd do that if they didn't think you were grown-up and responsible enough to take care of her."

"But I love watching Mariposa. And I care about the people of both dimensions and I want them to get along. It's the boring stuff I don't care about that's just so…you know…"

"Boring?"

"Yes! Boring! So very boring!"

Marco laughed. "Don't worry. I'll balance the check book and pay the bills when we get married."

Star smirked at him. "What's that, Diaz?"

Marco suddenly realized what he'd said and froze. "Oh…uh…well, what I meant was, um…"

Star sat up and pecked him on the cheek. "You're fun to tease. I actually kinda figured you'd volunteer to do that stuff. You know…" She leaned in close and whispered in her boyfriend's ear. "…once we get married."

Marco blushed, but his lips curled up in an involuntary smile. "C'mere, you." He pulled her close and gave her a long, passionate kiss on the lips. Their eyes met as they parted, and Star giggled.

"You know," Marco went on, "there are some perks to being an adult, too."

"Like what?"

"Well, now that you're eighteen, you can legally purchase a lottery ticket. And you can buy fireworks! …If you're in, like, Pennsylvania, Texas, or West Virginia."

"Why do you know so much about the legality of fireworks?"

Marco ignored the question, listing things off and counting them on his fingers. "You can donate blood, serve on a jury, sign legally binding contracts-"

"Not sure those are necessarily what I'd call 'perks', Marco…"

"You can open your own bank account, work at any job…" He stopped. "…And…" He lowered his eyes and smirked. "…you can legally do…that thing…"

Star frowned. "What thing?"

"You know." Marco waggled his eyebrows mischievously. "That."

Star's eyes widened. "You mean-"

"Ohhh yeahhh."

Star's face lit up with excitement. "I can purchase real estate!"

Marco nodded dramatically. "Uh huhhhhh."

"Wooo! Real estate! YEAH! I'm gonna buy me some LAND!"

This oneshot exists for the sole purpose of setting up that joke at the end. I made myself laugh out loud as I typed Star yelling "Wooo! Real estate! YEAH!"

I know there's no way Star would ever care about real estate, but it wouldn't be much of a bait-and-switch otherwise.