When you are running for political office, I believe you need to be prepared for the media to look at everything in your background, even from 20 years ago. Now, I do think that some items that are dredged up do not really matter much and the media makes a big deal out of them. However, the decisions people make can be looked at to see what kind of person they really are, not just what persona they show to the public.
If an event was 20 years ago is relevant to the debate, we should certainly talk about it. Someone's affair from 20 years ago doesn't count, but if someone has a history of different political views in the past than they do now, they need to explain that to voters.
Since it should be the voter's choice to decided they want to consider 20-year old events in their person decision making process ALL events, regardless of age should be factually reported during political campaigns. There should not be any restrictions on the reporting of events involving candidate's act in the public domain.
Since this question doesn't specify what events it means, it's hard to say, but any candidate should be able to address current issues that matter now. The state of things 20 years ago is not the same as it is now. If they want to address things that could be improved and use the past as a reference, that's another story.