Songs after 1922 can be covered.
It is important to understand Fair Use and how it relates to music and YouTube.
Here are sites I have found helpful.
http://www.copyright.gov/fls/fl102.html
There are four basic tests used to determine if copyrighted work can be used under the protection of Fair Use.
This site offers a good summation of those tests.
http://socialtimes.com/fair-use-youtube_b61891
So here's the thing. Most songs are originally intended as either entertainment, art, or biography.
So if your recording doesn't change the nature of the work, then you're in trouble.
But by TRANSFORMING the nature of the work into something else such as educational is covered as fair use.
So I highly encourage everyone to describe the transformational nature of their videos in the video itself AND in the video info.
Another easy way to stay off Youtube's radar is to NOT put the song name and artist in the video's title. Put something like Ukulele Underground Cool Cat Contest as the title.
Hope this helps.