They do age under tension but at a much slower rate. Usually after a week of regular playing my bass strings lose that metallic zing, but I notice if I don't get to play as much, they maintain that newly strung sound much much longer. Trebles for me last quite long even if played regularly - certainly longer than 3 weeks.
Also remember that the difference in sound could be a combination of factors such as humidity, temperature, your guitar "going to sleep" after not being played for 3 weeks straight, etc. Every guitar I have owned has responded to the environment, so my guitar has good days and bad days. It's the nature of wood instruments. Another factor is your ears may just be playing tricks on you as it is very difficult to truly remember what your strings sounded like 3 weeks ago.
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Excellence is available to all, but is accepted only by a few." - Christopher Parkening