That's true, I remember my friends and I being more excited to do Christmas things when we were in high school and college, especially because we often had to wait till Christmas break started (and finals were over) to really start celebrating.
Asa's understanding of human relationships is like Priscilla's understanding of clothing, makeup, and interior design...he can look at and identify things that are normal or desirable, but he can't correctly copy them, because he misses out on all the nuances. His "too old to decorate" thing maybe started with a kernel of truth...which is that teens and young adults don't participate in as many holiday traditions with their families as they get older. But that's not because they've grown out of being excited, it's because they've grown into having things like friends and responsibilities and jobs. None of which anyone in the Maass family has...and Asa isn't capable of paying enough attention to other humans beings to even begin to imagine an experience outside of his own, so he just chalks it up to a simplistic explanation of the kids being "too old."