This does not work. The arguments to `del` get appended at the end, after `find`. You want this:

  del() { mv "$@" ~/.Trash/ && find ~/.Trash -mtime +30 -delete }

I also replaced the unnecessary exec by find's builtin -delete

Correct me if I'm wrong, but wouldn't that instantly delete all moved files which are older than 30 days since mv keeps the mtime?

This fails on directories, as the -r switch was not included:

    mkdir foo; del foo

and including it will spawn file not found errors for the contents of the foo directory, if any.

EDIT: See also neighbor comment about appending filenames!

This is not fine, as deleting a small file can be very long if there's a large file to be deleted in the trash.