Executing program through Terminal
You need an alias. Typing alias sage='open /PATH/TO/Sage.app' will create an alias which will do exactly what you want, run the program just by typing sage. The problem is that this will only last until you close that terminal window (bash). So, you need to create that alias each time you run a new bash. To do so, just: 


Create or modify a text file called .bash_profile in your home directory.
Add alias sage='open /PATH/TO/Sage.app' on it
Save the file
Load the file once by typing . ~/.bash_profile


Remember that it will be case sensitive, so if you type Sage (instead of sage) it won't work.