Why does Capulet think it will be easy for Montague and him to keep peace?

Short answer: Capulet believes that because he and Montague are getting older, they are not so apt to start a fight as they were when they were younger. He says this in the first few lines of Act I Scene II, "But Montague is bound as well as I, In penalty alike; and 'tis not hard, I think, For men so old as we to keep the peace".
Background: There is an on-going feud between the two families and the Prince of Verona commanded a cease in the fighting. This is the bind he speaks about sharing with Montague in the first line quoted above. While the heads of the families themselves do not engage in fighting, the tension and violence between the younger members of both families cause the tragedy to unfold toward its inevitable conclusion.