Somewhere between our mid-teens and twenty is the age most people start thinking about what kind of adult they want to be. For a lot of us, that means coming up with a list of red lines we are determined we will not cross. “I will never become a capitalist slime bag that settles for a conventional life with a career and all that,” we tell ourselves. “I will never cheat on someone or cheat with someone”; and, “I will never become like that gross neighbour who is always drinking beer on Saturdays,” we promise.

And then life happens.

In the 1920s, Swiss psychologist Jean Piaget came up with cognitive and moral development theories. He found that by about the ages of 15 or 20 we move from adolescence into adulthood. Essentially, Piaget says this is the time in life when we stop viewing things in concrete terms, and develop the ability to adjust our beliefs according to our experiences. Many of our rules for adulthood make sense in theory, fewer hold up in practice, and others are just downright unachieveable. The difficulty is understanding what our teenage selves had right about navigating the murky waters of life, and what was just sweet naivety. How do we know what to let go under the steamroller of adulthood, and what should we cling to?

The conventional thinking is that we don’t know shit when we’re young, but come 30 we are suddenly full of wisdom. This results in copious lists of ‘20 Things I Would Tell my 20-Year-Old Self’. Most of these offerings are supremely unhelpful – there’s not much we can do to change things after the fact. The other problem with this line of thought is it ignores the ambiguities and nuances and utter confusion of being a grown up. And even if we were all given the handbook to life at the age of 15, it wouldn’t necessarily stop us all making mistakes. After all, the experiments and testing of boundaries are what make life, even if they go awry.

There’s also a fair chance we had at least a few things right as idealistic under 20-year-olds. Some of my own rules have served me exceptionally well, while I’ve ignored others at my own peril. Meanwhile, popular culture is full of tales of the 30-something who wakes up one day, depressed, to find they are just another rat in the race, having forsaken everything they promised themselves in their youth.

The cousin to this theory is the one that says there’s nothing you could tell your younger self, because you thought you had it all figured out and wouldn’t have listened. Note this exchange by Paul Hullah in The Leither magazine: “[Martin] often said intriguing things. When we were 40, I asked him, ‘If you could meet your 20-year-old self now, what would you say?’ He said, ‘I wouldn’t say anything, I’d just punch myself in the face.'”

The average 19-year-old is – admirably – adamant there is no way they would ever do X, Y, or Z, and you smile at them and say condescendingly, with a touch of cynicism: “Oh sweetie, that’s cute, I used to think that way too. But I can tell you, from my full 10 years of greater life experience, the world is not that black and white.”

Note also this reaction from a friend: “I had a list of ‘rules for my grown-up self’, like no drugs or drunkenness hahaha.” Much can be understood from this. Some rules are laughable because they’re not realistic: it’s not a given we will all forsake sobriety at some stage in adult life, but it’s statistically probable. And some rules are laughable because we’ve gone ahead and done the opposite, and in that hysterical giggle we may feel a twinge of nostalgia for the simplicity of youth – is it possible she was right all those years ago?

There’s also a fair chance we had at least a few things right as idealistic under 20-year-olds. Some of my own rules have served me exceptionally well, while I’ve ignored others at my own peril.

In the days before Piaget, there was L. M. Montgomery. As a child (okay, fine, at all times in my life) I loved Montgomery’s Anne of Green Gables series. The books were full of turn-of-the-19th-century advice about how one should behave as a member of society. In Anne of the Island, the protagonist is at university. On turning 20, she has a conversation with her housekeeper/chaperone/piss-and-vinegar-filled friend’s aunt, and recalls advice she received from her school ma’am back home in Avonlea:

“Miss Stacy told me long ago that by the time I was twenty my character would be formed, for good or evil. I don’t feel that it’s what it should be. It’s full of flaws.”

“So’s everybody’s,” said Aunt Jamesina cheerfully. “Mine’s cracked in a hundred places. Your Miss Stacy likely meant that when you are twenty your character would have got its permanent bent in one direction or t’other, and would go on developing in that line. Don’t worry over it, Anne. Do your duty by God and your neighbor and yourself, and have a good time. That’s my philosophy and it’s always worked pretty well.”

Be good to others and yourself, have a good time, and the rest will take care of itself. Solid life advice, that.

Fiona Broom is a freelance journalist roaming the world. She has been published in Al Jazeera, CounterPunch and Middle East Eye. Follow her on Twitter @Fiona_Broom.