What is the difference between 老X and X老 as a term of address?
Here's an explanation: In the case of people with good moral standing and high reputation (德高望重), you can add 老 after their surname, this is a very respectful (很恭敬的) address.

Screenshot from a book on Google Books. The relevant, very short sentence touching your question is highlighted (sorry, not very professional, but legible). 



Here's another source explaining this (mentioning scientists, statesmen, old teachers as the circle to whom you can apply this): 


  对德高望重的老科学家、老政治家、老教育家、老文学家，可以在姓之后加“老”字，以表示尊重。如“郭老”、“‘钱老”、“徐老”。