The mating behavior that early is usually a sign of dominance and taking over the lead in the pecking order. LF roosters are often mating pullets as early as 3 months of age even though the pullets aren't ready for egg laying. At this point I wouldn't think much about the behavior.
BackSwampGirl - egglaying starts as early as 5 months, with silkies laying later. MP is right that this is a game of establishing dominance rather than breeding. Bona fide breeding won't be possible until there's an egg to be fertilized.