6061 contains     0.60 Si        0.28 Cu       1.0 Mg        0.20 Cr

6063 contains     0.40 Si        0.    Cu        0.7 Mg        0.   Cr

I don't see how aging any amount of time can change constituants.

   devo: A friend of Mine who bends 6061 aluminum tells Me that  6061 T-4 can be bent when You first get it, but over time it ages and becomes more brittle, more like T-6, and can't be formed sa much as He needs to form it. I am not really familliar with 6063 alloys, but if the mentioned alloy ages and becomes more brittle, that may be what the author is trying to describe.

   I have to agree with Lawrence that the actual alloy is not likely to change with time, but the temper and formability might.