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.
