Writing lists to .csv file
Here is just a quick response to your code:


Look into the csv module – This module handles csv-files rather nicely
Look into using functions to separate concerns – At all the places you've added comments, you should/could use functions. And the comments should rather be docstrings on line after function definition like in the following:

def write_list_to_file(guest_list, filename):
    """Write the list to csv file."""

    with open(filename, "w") as outfile:
        for entries in guest_list:
            outfile.write(entries)
            outfile.write("\n")

Variable and function names are usually snake_case – See example above, which is according to PEP8
Use the with construct to encapsulate file handling – This is called a context manager, and it handles error cases, opening and closing of the file. See example above.
Why do you create the file, and not write to it directly? – I don't quite see why you do the # Creating .csv file, when you can both create and write to it directly.
Change text to avoid guestNumber – Having the guestNumber seems unneccessary. If you change the text, you don't need it. Using something similar to "Add another guest (Name, age, location) or 'Done': ". 
Doing capitalize() is somewhat "dangerous" – What if "John McEnroe" comes along, or "Jean von Luxembourg"?  They wouldn't like their name changed into "John Mcenroe" or "Jean Von Luxembourg". The same applies to the location part.
In general, don't add stuff you're going to remove immediately afterwards – That is I would most likely not add the Done guest for later removal, instead of adding it first and then removing it. Add a simple if in front of the append and the issue is avoided.
Avoid lengthy input prompts – Maybe personal preference, but I would rather have a lengthy text in front of the input prompt, instead of a lengthy prompt text. This way you could possibly also give even more examples of legal input.
Change into separate inputs – If you change to using the csv module, I would also change to three separate input statements, and possible do some simple validation on each of the inputs before adding them into the guest list.
Careful think about variable names – Some of your names are good, and some not so good. I.e. guestNames when it actually is both the name, age and location, is misleading. And in for entires in guestList, it would possibly read easier as for guest in guest_list:.

The latter is a typical pattern by the way, that is to use the singular of something plural, i.e. for guest in guests:, for number in numbers:, and so on. In general, plural version indicates list of some sort, whilst the singular version can be used within loops and similar.


I'll end this hasty review with a very useful construct which could cleanup your code (when you've created functions):

def main():
    """Main method used when called as a script."""

    guests = enter_guests()
    write_list_to_file(sorted(guests), "guestlist.csv")


if __name__ == '__main__':
    main()


Using this construct would allow for your code to be reused as a module later on, whilst still allowing it to be run directly from the command line. It also makes it very easy to understand what happens just skimming the code.