Tire replacement is really subjective. It depends on a number of factors. Five years is a bit short to me, though I know some manufacturers suggest that. It used to be 7 years for bridgestone but they have since taken that off.
I will say that I have about 6 years on my tires, the oem tires, and they do kinda suck now. The front is ok but the back is getting hard.
I had spitfires on my cb750 for almost 10 years and they were finally getting hard. Storage plays a big part along with temperature changes and, I think, UV damage.
I do not base things on cracks unless they are quite large. I have seen tires under 3 years of age develop cracks.
I guess for me, if you ride hard, 5 years. If fairly conservative, I would do 7 years max if it is a quality tire.
In this case, I would change them. I would never go over 7 years no matter how nice the tires looked.
I do respect others opinion though and there is nothing wrong with what they are suggesting.