You cannot rely on a visual inspection. Hoses, like most rubber parts, have a reliable service life of 5-7 yrs. You can always gamble and address each as the need arises. But, since you'll usually get stranded in the rain, or on the way to the airport, a date, or a job interview when one of them lets loose, replacing them along with the other work would get my vote. Also, be sure to include new worm-type clamps as Porsche recommends changing over to these from the oem spring-type if doing any cooling system repair.
