With both programming languages and spoken ones you also have the language moving on without you.
If you remembered pre-C++0x, or god forbid, pre-Standard C++, it would probably hurt as much as it helped, as your old idioms would either be outdated by new std features, or outright dangerous by modern coding standards.
At least with old spoken languages, you'll just either sound like a small child or a character from an old movie, using slang that no one uses anymore.
It would not be useful, unless I lived in an area where I would use it. Maybe if I moved to Canada.
Same goes for computer languages. At one time, I was quite fluent in C++, but I can hardly recognize it, anymore.