What age does a foul become horse?

There's no specific age at which a "foul" becomes a "horse." This is because those words are not related in terms of animal life stages.

"Foul" typically refers to something unpleasant or offensive. It can also be used in sports to describe a rule violation.

"Horse" refers to the large, domesticated animal.

It's possible you're thinking of a different word or phrase. Can you clarify your question?