I really don't have a problem with age, or in the author's case, any specific type of jersey. Wear them at any age, wear any type. But please, please, please, do so at the appropriate times.
Do not wear your Garnett jersey, no matter what age to a christening, bar mitzvah, any family function not in your own home, job interviews, etc. Yes, I once interviewed someone for a job that came in wearing an Iverson jersey. Refrain from wearing that Brady jersey out to a nightclub, on a date, or to any restaurant that isn't a fast food joint. Do not wear your Papelbon jersey on the day you have to get your license renewed or school pictures, or for holiday pictures.
Have some semblance of decorum. I own a Garnett jersey that I wore last year for playoff games I watched at home. I have a Brady jersey for games I watched at home. If I went to a Superbowl party where the Pats were playing, I wore my jersey. Around the house, not going anywhere, or having a barbeque, I wore them whenever I wanted.
I'm 43 years old. I have no problem saying I admire Garnett's abilities even though he is younger than I am. I have no problem showing my allegiance to my team and Garnett by wearing the jersey. I don't see age of the jersey's owner as mattering.
But where one wears the jersey is crucial. If I go to a Pats game(haven't at Gillette yet), I would wear my jersey. I would wear my Garnett jersey to the Garden, but it is an authentic St. Patrick's Day green and gold one that cost my son a small fortune and too many people like to spill stuff at the Garden, so I won't where it there.
BTW, I thought Dick Vitale looked pretty [dang] good sporting that Rays gear. Good for him!!