If I have my way..........never.
Look at my signature and you can see I certainly don't get tired of it even as a 30 year old man. In 2009 I went with my wife (her first time) and it had been 7 years since I had been. I adored the parks growing up and the first thing I told myself was:
"Look, don't get upset if the parks just don't have that same feel anymore, you're 29 years old now and maybe you've changed."
haha, our first day we went to MK and those old feelings came back. I could barely stand still waiting for the monorail to come and pick us up. Here I am this big football player-sized man just dying to walk down Main Street again. People at work occasionally bug me about loving WDW so much but it doesn't faze me at all.
So to answer the original poster, all I can say is that I hope I am never too proud of a person to openly enjoy WDW. I have a feeling that even if I am elderly and have a hard time walking that I'd force myself to go there