Sam's been sixteen years old for a little under four minutes when he feels a warm breath along his neck. Half awake, he rolls over in the direction of the warmth, smiling slightly, and hears his brother's low chuckle.
"You're legal now, Sammy," Dean murmurs quietly. "You know what that means?"
"Are you about to take my innocence?" Sam asks with a grin.

It's been a year and a half since Sam confessed his feelings - well, that's what Sam calls it; Dean tends to jokingly refer to it as sexual assault - and once he'd gotten over the initial shock, Dean had realised that his own feelings towards his Sammy were somewhat more than brotherly. Since then, it's been a flurry of stolen kisses, hurried handjobs and haphazard blowjobs (both boys got a secret kick out of the possibility of discovery), but there's one thing Dean's always insisted on: no sex until Sam turned sixteen.

"It's illegal, Sammy!" Dean would always laugh when Sam pressed the point.
"Whereas incest, yeah, no worries there, right Dean?" was the invariable response, which just made Dean laugh harder, and invariably do something very distracting with his hand. But now Sam was sixteen. And although he hadn't expected Dean to be, well, quite this eager, he had to admit, he was looking forward to making all those months of stored-up fantasies come true.

By this time, Dean had emptied out his pockets - Sam had to muffle his laughter when he saw the condoms and lube - stripped, and hopped into the double bed. It was an uncomfortable fit at first (seeing as Sam had grown about a foot in the last couple of months), but Dean quickly rectified that by wriggling in between his brother's legs and licking, kissing and nibbling his way down Sam's torso. Sam has to stuff the pillow over his face to stifle his moans - as amusing as the mental image is, this isn't something either of them want their father walking in on.

Dean's had a lot of time to work out Sammy's hot spots, and he takes full advantage of it now: teasing a nipple, running his tongue down Sam's chest towards his navel, nipping the skin at Sam's hip... and all the while, Sam bites down on his pillow and clenches his fists to stop himself screaming out his brother's name.

Dean's enjoying watching Sam squirm, but he's been waiting as long as his little brother has for this moment, and before long he's reaching for the bottle on the bedside table, panting heavily. He's got the bottle open as he murmurs, "You ready, Sammy?"
"When you are," his brother responds lovingly.
"Okay," Dean whispers, as he moves his lips along Sam's jawline. "I'll top; you bottom... this'll probably be a little cold - what's wrong?"
Because Sam's suddenly gone very, very still, and is starting to sit up.
"You don't... Dean, you're not topping. No. No way."

A whispered but furious argument ensues. There's a muttered "I'm the oldest" and a plaintive "But it's my birthday!", among other things, before Sam leans in to kiss his brother; soft and slow.
"There's one thing we're not taking into consideration here..." he whispers softly.
"What?" Dean asks. It takes him far too long to notice the mischievous grin on Sam's face; by the time he realises, Sam's already flipped him over, and grabbed the bottle of lube.
"I'm stronger than you," he laughs down at his big brother.