People are always moving around, and it it sometimes very difficult to find someone if all you have is an old email address.
There are still some mail lookup services that you can use such as emailchange.com, bigfoot.com and reversepath.net. If you have changed email addresses, and would like for old friends to be able to look you up, it would be a good idea to register with each of them.
emailchange.com and reversepath.net old email address lookup (Free):
Bigfoot old email lookup:
FindMeMail old email lookup:
It would be a good idea to register with these databases so that old friends and potential employers can find you if they need to.