1. Doing taxes is not a project you work on somewhere between January 1st and April 15th. You spend all year dealing with it, and it is mind-numbingly dull and tedious.
2. The compassion of your creditors should never be counted on as an option when you get behind. This is especially true of medical bills. Get a credit card exclusively for these expenses, or have a substantial savings account.
3. Stuff breaks down constantly. Big things like the car or the water heater, less important things like your mobile phone or your hair dryer. I have come to believe that at least 30% of waking time is spent in fixing things, or saving up to replace it or pay someone else to fix it.
4. Pets and kids are capable of making more messes than you could ever possibly believe. Get used to it. Develop a sense of humor about it, or you'll go crazy.
5. No one's life really looks the way it does on social media. Cropped out of that photo is the garbage can the dog knocked over, or the dishes molding in the sink because you've been too sick to deal with them, or the pile of dirty laundry that changes size but never truly goes away.
6. Yards take a LOT of work. If you don't like yard work, find a place to live where you don't have any that needs doing, or make sure you can afford to pay someone else to deal with it.
7. There will always be someone who wants some of your time. If you don't schedule it, someone else will.
8. Sure, raising kids is rewarding, and totally worth it. It's also a huge challenge. Toys will get stepped on, there will be injuries, various bodily fluids will end up on various surfaces, there will be whining, crying, tattling, lying, tantrums, and bad attitudes, no matter how well you think you've raised your kids.
9. You will get grey hair and wrinkles, and you will one day find it takes so much more work to get/stay in shape.
10. There are a million ways to waste away the years, and you'll succumb to them if you're not vigilant.