Roosters may start mating at at leats 2 months old.But,they are not fertilzing any thing.Roosters can fertilze at 5 months.
Any egg being made or made,as soon as the rooster has mated then hen won't be fertile,but any egg that is made within the hour,or the next day,probably will be fertile.
Most hens can be able to be mated with at 5 months.