It always seems to me that Mandarin spoken by older generations is a lot harder to understand than the Mandarin spoken by younger people, even if they're not speaking their local dialect. I always assumed it was down to the teaching of "standard" Mandarin not being as widespread and comprehensive than it is now (though I still vehemently protest at there being any concept of there being a "standard" in any language)

It's as different as a 1 day old apple to a 2 day old apple.

It's as different as white on rice to white on snow. 

It's as different as a 50 kuai basketball to a 60 kuai basketball.

Yeah, not much difference at all just some small differences here and there. 

Pretty much the same.  Just like English in the 60's and now.  Some phrases have changed, era specific words have gone out of usage (remember the cultural revolution began in the 60's).  Apart from that it's the same language.

I cannot say for spoken, but I know that the written language underwent massive changes in the 1950s, when Zhou Youguang helped developed "simplified" characters. 

http://www.bbc.co.uk/news/world-asia-china-17455067

He's 107 years old, still alive and kicking.