Would it be too late to start learnings music at the age of 20 so that I can play well enough to be in a band?

It depends on what type of band your looking for. If you wanted to join an established band a minimum of 2 years would probably be sufficient. But people also learn at different rates and not every instrument is created equal. Some take longer to master. Or even get the basics down. No 20 is not too late but you better be dedicated to your instrument. Because a lot of people who are playing in bands started way before you. But you may suprise yourself and pick your chosen instrument up fairly quickly. You are the key to all this, you put in what you get out. Good luck and remember that there will always be chances to join a band. Music is universally liked just about everywhere. You will have plenty of opportunities to try out and make one. Hell you may even start your own one some day. Just dont lose sight of your goal. Also not all categories of music are created equal. Some are harder to learn than others. You need to start by picking your instrument of choice. The music comes next

Not at all. I'm 23 now, and I play the 5-string banjo. I started when I was 18, right after high school.

Well, there is no right "level of music" to join a band. If you want to be decent enough to be in a band that the general public would consider GOOD, I would say AT LEAST two years of practice would be suffice. I've been playing an instrument for 8 years now and I still have lots to learn.

Of course it's not too late. There's plenty of famous musicians who can't read a lick of music ect.... as well as tons of "professional" studiers of music... IE Berkley college of Music students who don't make it big in a band. You just have to find the shortcuts and take them.

It is never to late

not at all. you are still very young, go for it