Is 17/18 too old to start piano?

No it's not too old. You are 17, not 70. And even 70 isn't too old. Something you have going for you is that you WANT to learn to play and many kids were taught whether they were interested or not. It does take time to learn but at 17 you have a while lifetime ahead to learn. My brother started piano lessons at 40. My father started when.he was 65. Both became pretty good. I started guitar at 50. No one is ever too old to start any type of music lessons. 

As for starting, there are websites, YouTube videos, and more that cover the simple stuff.  There are phone apps that turn your smartphone into a piano keyboard so you can learn and practice some simple things before spending a lot of money. A piano key is very different from a touchscreen but the theory of music isn't.  Start with the ABC's of the white keys and the sharps and flats of the black keys. Learn the basics of whole, half and quarter notes. Then read up and learn about scales. When lockdowns are over find a teacher and you will already know the basic stuff.

It is NEVER too old to start anything. Yes, people may judge you, but it doesn't matter what people think, it matters if you're happy with the results of your actions in your life. You wouldn't be friends with the people that judge you anyways, so don't assume what they think in your mind and dictate your actions because of that assumption.

No age is too old to start piano. 

When I was a child, the prevailing "wisdom" was that kids should be 4-5 years old to learn an instrument. 

I started violin/viola at 65, and worked my way into a local orchestra. 

People start when they are a lot older than you.  Most people learn faster and more correctly with a qualified teacher.  Structured lessons will develop your fingers properly.  When it gets difficult, don't give up.  Anything that is easy is not much of an accomplishment.  I have not made much money with my musical abilities, but I do not regret a minute of all the hours of practice.

No, it's not too old to start learning piano.

When my daughter was about 5 years old I would always give her one of those toy electric organ pianos for her birthday each year I would get her a better one and a better one she played with it. But she never really thought about taking piano lessons.

At the age of about 29 years old my daughter who was attending the Pentecostal Church holiness. Suddenly became interested in playing the piano for the church.

The holy Spirit as she said taught her how to play.

And my daughter plays with both of her hands and her fingers like a real pianist and nobody ever taught her except herself in the holy Spirit

It's never too late but it's just a lot easier if you start as early as possible. People are the quickest to learn if they start at age 5 and 6.

I think I was 53 when I took my first lesson. A local non-profit radio station was having a fund drive, and a local Music Therapist donated a 30-minute lesson. (Told her I wanted to learn Maybe I’m Amazed. That might have been a tad ambitious for a beginner, lol). Definitely an experience to learn from not just an educator but a head-space practitioner. Yeah. Do it.