I’m with Whale on this, if it ain’t broke, don’t fix it. There are a lot of cowboys out there in the roof restoration industry. It’s worth looking at the Monier website, according to it concrete tiles actually become less porous as they age.
Also look at the Whirlpool forums on the subject.
Whilst older rooves lose their gloss and street appeal, coatings and paints don’t usually last very long in such a harsh environment. Sure they look great for a few months or perhaps up to a year after application, but they start to break down after that and ultimately often look worse than if you left the roof alone in the first place.