For car tires, the maximum "safe" life is six years from the date of manufacture. So if they are older than that, replace them whether they look bad or not. I suspect scooter tires are the same. But I'm not certain.
The rule of thumb is to not purchase a tire that was manufactured more than 2 years ago. But in reality, I'd buy them as old as three years old if I could get them at a substantial discount and I knew that I'd wear them out by mileage before 6 years.
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