Dripping with sweat. My main memory seeing in my 20th birthday clubbing with new uni friends. Then like that...BOOM, I'm proposing in Central Park days before the big 3-0. A decade of finding my feet and now youth traded for responsibility - which in this sense I'm gonna define as either, a mortgage, kid or marriage.
Weekends spent wrecked on a friend's couch, swapped for wholesome brunches and budgeting - where just finding the exit of IKEA can be the highlight! OK so a slight exaggeration. I still enjoy the odd session.
I've loved the last 10 years. Parties I can remember, nights I can't, finding my future wife and even stints of unemployment which made me value the jobs after.
Whether with your career or friendship groups, change will be inevitable. What would I want my 20 year old self to know? What would I even listen to?
I'm not sure, but given a Back to the Future scenario I'd go for something along the lines of:
Starting out
Move fast, break things: In the decade of social media it seems only fitting to borrow Facebook's early motto. Knowing what you want and what you don't comes from failing, so to quote Samuel Beckett 'fail better'. Screwing up is fine, there's plenty of time in your 20's, just preferably don't make the same mistake twice. Translatable experience is increasingly sought after. So try things out and quit if it's not for you. Quitting, unless it's smoking cessation (something advisable before 30), is not a dirty word.
Be adaptable: You learn nothing from doing the same thing well. Challenge yourself to feel awkward often and it will become easier to deal with difficult environments, you know like Christmas hungover with children and in-laws.
Automation of the nation: OK, so you might want to pick a career that won't be ousted by technology in your lifetime, think what won't the robots be able to do (come the singularity)...what is the most human of all roles? Wine taster? Director? Diplomat?
Professionally
Confidence overcomes experience: Age brings experience and the realisation that no one really knows what they're doing out there - but you become better at masking it. Confidence and a reasoned argument behind your thinking, is usually enough to get you through. Confidence breeds trust, but if in doubt follow your gut, for more advice on that topic read Blink (Malcolm Gladwell).
Never stop questioning: Let's say bluffing isn't working. What next? Ask those who know more. Simple. There will always be someone better positioned or more intelligent to ask for help, especially at the beginning of anything new. Ask challenging questions to those who are more expert than yourself. Insecurity may creep in and wonder if this will make you look stupid. Usually it won't. People like to talk and a good question will display understanding and show you value someone else's opinion.
Grow network: Every year you meet hundreds of people, many of whom can potentially aid future-you (and vice versa). Stay in contact regularly enough so it doesn't seem weird when you come to call on them.
General
Shit can and will happen: It doesn't matter how you prepare for events, they will never turn out as expected. But focus on the elements you can control and disregard time wasted on those you can't - that doesn't mean ignore them, just manage their impact on yourself. Consider the effect, on your own happiness, an increase in control could have, even if it's just perceived.
Regret less: Drinking half a bottle of tequila, spilling red wine on a white sofa and buying said white sofa; there are many things you'll blame yourself for. But spending time regretting decisions is pointless - apologise, show remorse and move on. I've tried to avoid clichés (another recommendation), but youth is wasted on the young, so enjoy it, take up offers, especially opportunities of travel. Holidays with friends will decline with age, and those you had, will be nostalgically remembered years down the line.
Be nicer: It is very hard not to succeed more by just being nicer. One friend swears by Wilson Mizner's mantra 'Be nice to people on your way up because you'll meet them on the way down'. Even if it's selfishness disguised as altruism, you will generally get more time from people and achieve more when reviewed personally or professionally.