In a couple of months, I will turn 40 years old. When I was a kid, that was the age that seemed impossibly old. The age after which we weren't supposed to trust people, the age that was "grown up" — to my mind, at least.

Of course, now that I've almost reached that age, I realize that's not the case. So, what have I learned? My first golden rule is this:

1. Plan for the future you want, not the one other people tell you to have.

I wasted a lot of time when I was younger trying to fit into other people's ideas of my future instead of planning for the future I wanted.

Here's a practical example of how that plays out. The typical advice is that you should start saving for retirement in your early 20s, or by your 30s at the very latest. I did that, diligently saving up, starting with my first job because I was told to.

Now, that can be great advice and it's smart to follow it — but only if you see yourself wanting a straightforward retirement. Do you want to work hard, full-time for 40ish years and then quit altogether? If so, save away (and start ASAP).

But I made a different choice, for two reasons:
1. All the jobs that would have allowed me to save for retirement and retire at 65 meant I would have had to sacrifice the life I wanted to lead.
2. I don't ever really want to not work. I want to keep writing, editing, teaching and doing small, interesting jobs on the side until I die. Taking jobs that I don't want so that I can retire and do what I "really" want doesn't make sense for me/ I'm already doing what I really want to do.

So, plan for your financial future — but take into account that your future may not look like your friends' or your parents' older years. This applies to things other than finances, too. Do you want children? Do you want to get married and have a big wedding and honeymoon? If not, your 30s and 40s will look different than the traditional path. If you do want those things, start preparing for both of those eventualities now; babysit for friends' and neighbors' kids so you can learn about how to care for kids; prioritize finding a mate so you can have time to prepare as a couple for kids.

Personally, I chose to neither marry nor have children because I have other priorities. Remember, there's more than one way to live an adult life. Choose what's best for you, and prepare for it.

2. Take care of your body

People cooking and plating food in a home's kitchen Learning how to cook healthy meals isn't just about a healthier you; it's another way to spend time with friends. (Photo: Monkey Business Images/Shutterstock)

Regardless of any other life choices, you want to be as healthy as you can be now and into your future. By your 30s, you should have put smoking cigarettes behind you, and have figured out a workout plan that you can stick to. You should be able to cook or prepare meals for yourself that are healthy and satisfying. Your health care should work for you and involve some preventative care — you're too old to be seeing a health professional only when you get ill or hurt yourself.

3. Know what brings you joy

What do you love to do? What makes you happy is a key facet of adult life — not so you can indulge in it all the time, but so you have a toolbox that you can reach into when you need to. Look past the superficial answers here (cocktails, a great TV show) and dig in. What brings you deep-down joy? If you haven't discovered this by the time you're halfway through your life, you are missing out. Which leads me to:

4. Know how to pull yourself out of a rut

Awful things will happen in your life. It's true for everyone, and I'm sorry to tell you that. A rip-your-heart-out breakup, the loss of a family member, a dismissal from a job, a car crash, or even a series of low-level frustrations will bring you down at some point. What can you do to pull yourself out of that negative place? What are the things you can do to care for yourself? Who can you lean on in hard times? This is a life skill that few of us want to think about but one that's invaluable when you need it. By the time you turn 40, you should have some ideas about how to recover from setbacks and grief.

5. Be able to say 'no'

Being an adult means that sometimes you need to put yourself and your work ahead of other people's needs. The best way to know when and how to say no is to practice it. (And yes, it does get easier over time.)

6. Take non-work time for what you care about

Woman in hat smells a flower and enjoys nature At this point in your life, you should have expanded the ways you spend your time beyond work. If you love nature, then find a way to make it a part of your week, every week. (Photo: Sjale/Shutterstock)

By the time you hit 40, you should know what you love outside your work life, and be doing it (even if you adore your job). Whether that's spending time with your family, traveling, volunteering with an organization whose ideals you support or learning new skills in your favorite hobby, a well-rounded person doesn't just work.

7. Know your boundaries and enforce them rigorously

This is honestly one of my favorite things about being an adult. You are now empowered by age and experience. You don't have to take anyone's abuse or cruelty; if friends or family members aren't treating you with love and respect, speak with them about it. If they don't stop their behavior after you've made clear that it hurts you, walk away from those relationships. There's no room for toxic relationships once you're an adult, and boundaries are for you to set and enforce. It's OK for some people to disagree with those boundaries — they can make their own.

8. Show up for the people you love

It's easy to be flaky when you're young, and most are forgiven for it. But once you're through your 30s, it's time to stop ditching friends because you're too busy or ghosting from a party. Be there for the people you love when they need your help, your company or a shoulder to cry on. Thank your hosts when you leave their party with a handshake or a hug. Send thank-you notes (email is fine); send flowers (or plants, or chocolate); RSVP; and show up if you have RSVP'd, even if you aren't feeling yourself that night. Taking care of yourself doesn't mean you get to disregard other people or treat them shabbily.

9. Make time for yourself — and be specific about what that is.

No, I'm not talking about spa days. Sure, those are nice, but why is that the only thing people think of when they think of taking personal time? Whatever it is you need to do for yourself, do it on the regular. As 60-year-old reader Nancy told writer Mark Manson, "... do something for yourself every day, something different once a month and something spectacular every year.” That's a solid goal.

10. Accept that even the best-laid plans won't work out

You don't have control over everything, and by the time you're 40 you will have realized that — or made yourself crazy in the process!

Starre Vartan ( @ecochickie ) covers conscious consumption, health and science as she travels the world exploring new cultures and ideas.