They can when they're teenagers, however in most countries, this is considered illegal.
Sexual intercourse is defined as consentual physically intimate relations with another. As children do not have the true capacity to offer consent, it is generally viewded that they cannot. While most children who have reached the stage of physical development where secondary sexual characteristics would physically allow for it, it is inadvisable to attempt, as it can easily be seen by law enforcement as coercion, or even a form of sexual assault. That is why states have an 'age of consent'. Without legal consent, it isn't intercourse, it's a criminal act.