You don't have to be 12 to own sports gear of your favorite team. Many adults of all ages wear jerseys of their favorite player or championship gear or just team pride. There is nothing childish about it.
Not childish...but a big waste of money.
Why? Gotta wear a shirt or a cap, why not make it one with your team's logo?
And yes, FTR, I own a Crosby Winter Classic jersey (my pride and joy, a 40th b-day gift from my sister), more Pens t-shirts than I can count (think one is Lemieux from when he was still playing), a long-sleave jersey, a golf shirt, a scarf, PJ bottoms, slippers, and a ball cap or two. Most were gifts.