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

The answer to this question can be found at the very
beginning of Act I scene 2, as a conversation is continued between Capulet and Count
Paris regarding the feud and what has happened between the two households. Having
received such a stern reprimand from the Prince, however, the worries of Paris which are
obviously referred to in the response from Capulet are downplayed. Note what he
says:

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.

Thus Capulet considers
that both households are under the same restrictions--no favouritism is shown--and thus
both know the price of breaking the peace. In addition, older men such as Montague and
Capulet, in his opinion, should find it easier to keep the peace than younger, more
intemperate individuals. Ironically, he is right, as the rest of the play will
testify.