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When I was a kid (10 to 14 years old) I was taught to play the piano. Or that's what I thought until today. I haven't been playing it for about 10 years so I am of course rusty, but what I noticed is that when I learned I had developed bad practices (for example constantly looking at the keys instead of looking at the sheet). Moreover, I wasn't taught anything at all about chords, scales and so on. So what I did back then was playing pieces (I remember playing a Ravel piece during my last year) by heart without particular knowledge of what was behind them. I'm trying to start playing again but now that I'm older (and my learning is less elastic) I find it really hard not to get discouraged. I think I should go back to the basics, but I'm a bit confused on where to start again. What theoretical concepts should I cover to fill my knowledge gaps? Is it a good idea to learn with a teacher?
Thank you.

Lets start with the obvious: you will do better with a teacher than without a teacher. This is for many reasons but two of the main ones are that it will bring structure to your learning and development and you will get regular feedback on how you are progressing.

If you cannot get a teacher then you are going to have to be very disciplined. If you were playing Ravel when you gave up previously then you must have made some progress back then (I can't think of any really easy pieces by Ravel) but that is no longer your starting point.

Start with the basics and move forward only at a pace that does not skip things. Its obvious I know but its amazing how easy it is to move on from a piece when you can play most of it and you intend to "come back to that hard bit later". Usually does not happen.

If you get it right then you will learn about different keys and chords as you progress. If you end up without a teacher then try to get a look at the syllabus from one or more of the music schools (ABRSM for example). Their grade 1 to Grade 8 pieces represent a reasonable line of increasing difficulty and complexity, as do their scale and arpeggio requirements which go alongside them.

You have already recognized that you have some flaws in your approach (you mentioned looking at the keys) so that's a positive. You have recognized an issue which is halfway to fixing it.

Go for it. Good luck.

First of all, I recommend not thinking about your age or the "elasticity" of learning now. I got interested in music when I was 14, and in the country where I lived at the time I would not be accepted into anything but percussion, because I was considered too old (and even that came with noticeable displeasure from my teacher).

I had played percussion (drums, kettle drums, xylophone) for 8 years in a folk orchestra (so no classical music or jazz). I was very careless with practising and rarely played outside of the 3 3-hour-long rehearsals per week with the rest of the orchestra. After a few years into university, I completely gave up on music (I didn't have the time to come to the orchestra anymore, and I didn't have much money to eat, let alone purchase a drum set).

I started to play saxophone (tenor), my favourite instrument, just around the age you are now. Most people in my place would've given up on the idea as soon as it arrived in their mind, but I had an almost lifelong passion for the instrument, and having worked for a number of years after finishing university allowed me to buy a professional instrument and enrol into a semester at the local conservatorium.

Because I was older, it was actually much easier for me to follow a practising routine than when I was a teenager who was mainly concerned with getting good grades and staying in touch with their friends. Investing a car's worth of money into a professional-grade instrument was a great incentive to keep on practising too.

As @JimM said, it is a good idea to invest into lessons. If you practise for 25 minutes 2 times a day and see a teacher every week for 3 months, it will pay off much more than you possibly expect now. Just make sure you do not overexert yourself in the beginning, as it can lead to long-term physical damage to your fingers, hands and posture.

When I came back to playing music, I could barely read note sheets. I started with simple pieces (like Mozart's minuets) and had to write note names over the staff to play at all. Now the saxophone is in many ways easier than the piano, so you might have to separately exercise with the treble and bass clefs in the beginning, but do not let it deter from your desire to learn to play the instrument again. You have most likely retained some physical memory from playing it as a child, which will come back to you eventually.

I know it is daunting at first, even financially speaking, but if you love the instrument and truly want to master it, you will succeed. It took me 1.5 years to go through 10 years of school-level saxophone, and I am hardly a genius at music, so I'm certain anyone can repeat my "success".

Please do not forget to research instruments and how to look after them prior to engaging. You will eventually have to get an instrument of your own. If you have a local conservatorium, it might be a good idea to enrol there, they usually offer instruments for hire, so you might be able to get yourself some time to find out whether you want to be bothered with the instrument at all before you fully invest yourself into it.

As for chords and music theory, those concepts come in time as well. If you get a good teacher, you might ask them to help you analyse the compositional techniques behind a particular piece. Most (good) piano teachers mention the concepts of tonality and harmony right off the bat anyway, but it's a good idea to ask for additional information and books. As you get deeper into playing piano and progress to more difficult pieces, you will start noticing patterns yourself. Just give yourself time and do not expect to grasp everything at once.