I think this is incorrect, but maybe others could chime in. With AMEX, they report the age of all your accounts the same. If you've continuously been a customer since 1998, it will say that on all accounts. If you cancel, it should still say that. With that said, you will lose one card with an age of 14-15 years. You could always get another AMEX, maybe the SPG, and still have the same age, etc.

Also, I dont think it will fall off your credit beureau report for 7-10 years even if you cancel it, so won't affect your credit anytime soon.