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Me? I just openly play. I have actually started my friends back into playing Pokemon. Wish lists now contain pokedolls and old carts we're missing. One of us even carries that HUGE pokeball case from Target to work.
By the way, we're all adults over 30 who live on our own, are in relationships or have been in relationships, have jobs, and do things with our lives. Pokemon is just a fun way to kill some time and have yet another thing to talk about. Some people blab about Game of Thrones, we're talking about how much we miss Brock.
...and nobody cares. Seriously. Either that or they LIKE that we're playing Pokemon.
The guy with the pokeball case works as the head chef at his restaurant. A few co-workers have picked up pokemon again to try to beat him. The owner thinks it's cute to get everyone together after work for beer and imaginary animal fighting.
Another one of us runs his own business and carries a Spongebob Gameboy Advance SP with him. He's doing Nuzlock Blue. His clients always get nostalgic and happy when they see his Gameboy and the game inside. They see it as wholesome that he plays innocent games for fun instead of drinking or drugs.
I randomly play with anyone I meet. Streetpass turns up 30+ people on a regular day for me as a traveling tech. It's great for killing time while waiting for scans to complete. I have sold quite a few adults on the idea of getting a 3DS (usually with pokemon) as they actually like what they see when I show them mine.
> TL;DR: Don't be scared. If you're not weaboo about it, people either won't care or will join you!