Well from the Kato interview...
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4. Throughout the game, whenever characters from Chrono Trigger are mentioned, they are talked about as if they are dead. I can understand this with Lucca. However, shouldn't Crono and Marle still be alive? This game takes place twenty years after the plot action of Chrono Trigger, correct? Then, why are the Chrono Trigger characters talked about as if they are dead?
Question From: The Mad Druid spiralwalker@hotmail.com
This is another difficult question. (laughs) The fate of Crono and Marle is not thoroughly explained in Chrono Cross, but taking into consideration the fall of the Kingdom of Guardia and the rise of Porre's militarism on the main continent, there is a good chance the two friends may have been involved in some kind of incident.
He never directly states they died, but I think he strongly hinted it. Either they are dead, or injured beyond repair. >_>