This Stack Overflow article covers the subject well:

http://stackoverflow.com/questions/4561895/how-to-recursively-find-the-latest-modified-file-in-a-directory

This is a slightly modified example:

    $ find . -type f -printf "%T@ %p\n" | sort -n | cut -f2- -d" " | tail -7
    ./dir_a/README.md
    ./dir_a/hello.java
    ./dir_b/README.md
    ./dir_c/README.md
    ./dir_c/subdir/src/Hello.js
    ./dir_c/subdir/src/Goodbye.js
    ./cached-dir_a/README.md