Men who refuse to buy feminine hygiene products for their wives/girlfriends need to grow the fuck up! No one in the store is going to think they're for you, and you're not important enough for the clerk to care what you buy anyway, so grow the fuck up.
Women who freak out about stretch marks need to grow the fuck up! You're having a baby and your entire life is about to radically change in just about every way imaginable and you're concerned about next year's bikini season? Get over yourself and grow the fuck up.
Stepparents or live-ins who think that by virtue of being older they somehow have instant parental authority over the kids need to grow the fuck up. Fine, you're an adult and it's your home so you can set some rules, and when you're alone with the kids you get the babysitter power, but otherwise you're just Mom or Dad's spouse/live-in, not an actual parent. Your job is to back up the real parent, not to step in and pretend you have the authority that comes with having been there from the start. It doesn't matter how much you want your friends to respect you for taking on someone else's kid or how much you want the former single parent to see you as their savior, the kid's not going to fall for it and the hero worship you expect ain't coming anytime soon either, so grow the fuck up!
Parents who want to be "cool" need to grow the fuck up. When I was 16 I swore I'd never be as strict as my mom was but I know what I got away with and now I swear I'll be twice the bitch she was to me. Buying beer for the neighborhood kids or letting your kids have sex in your house doesn't make you cool. At best it makes you a joke to the people your age who've actually matured in their lives and at worst it makes you a flat-out bad parent. Maybe they'd do it anyway and you'd just rather they did it at home, whatever "it" is, but the fact is that they'd do it a lot less if they had to go to some trouble to get away with it. Put your daughter on the pill and give your son condoms, but don't sit at home with the TV volume turned up so you can hear it over the party in the next room! No teenager is going to name their neglectful parent as their best friend anyway and any authority you might otherwise have had will be washed away with your horrible displays of judgment, so just grow the fuck up!
Adults with kids who can't prioritize need to grow the fuck up. If you've got 2 kids who can't afford school clothes in the fall but you've got a new tattoo and a Harley, you need to grow the fuck up. I don't care how nice your hair looks with 3 shades of highlights or how sexy your man looks in his leathers if your kids are living off of mac and cheese and state-funded school lunches. It's a lot easier to respect someone who sacrifices it all for their kids than one who works overtime to go out drinking with friends while the kids sit home in a trailer with a bad roof, so grow the fuck up!
People who think they're better because they have a "clean" job and you have a "dirty" one need to grow up. If you have a drug conviction and 2 DUIs, I don't care how white your collar is compared to my truck-driving husband's. The label on your jeans or the price of your haircut don't mean shit to some people and believe it or not there are people in the world who don't keep score. If you really want to know who "wins", see who's happiest. It might be the guy with grease under his nails and not the one with the corner office. If you think status is everything, you really need to go grow the fuck up!
People who try too hard to be different need to grow the fuck up. I was one of them back in the day but guess what, I needed to grow up! If you have to spell your baby's name Mickaeylah and over-complicate things just so people will know you're "edgy" and "unique" then you're not. If you have to have 3 different colors in your hair (black, blond, and pink?) just to stand out then you don't. Want to really be unique and different? Take up quilting, or learn to square dance. THAT'S the stuff you don't see every day. But trying to stand out by doing what everyone else is doing just shows you need to grow the fuck up!