Modern Tires

OK- I'm an old fart that needs to be educated about modern tires. Went to a tire shop yesterday to get a flat repair. They wouldn't repair it because said that the tire was too old! They are 6 years old. I am hearing this more often- that our tires are old if we see some aging (cracking) and they need to be replaced. Am I to assume that the quality of tires recently has gotten so bad that they need to be replaced because of "old age"? Seems like an excuse just to sell more tires. Am I wrong? Please educate me.

If tubeless I just plug my own-----