You never grow out of interests but you can lose interest which can be because of many reasons, trust me im 20 almost 21 and people may consider me too old, but if you saw me in IRL you would never of thought i would play Pokemon or even watch Anime.
I think i always kept my interests my interests and never lost in them, i dont like stuff because other people do, if i got a keen interest in that i will stay interested regardless, Pokemon is one of them.
I am a 90's kid and grew up with Pokemon, Cards, Anime, the days where you felt so good to see your hall of fame on Pokemon blue, beating red on Mt Silver, memories stay with us.
But i do think i will always stay interested in Pokemon and its games, its not because im childish.
Ask many people a generation before us and ask if they still love Mario on the snes, they will say yes, My Nan plays the Wii because she loves Mario, as well as My uncle. Its just what we was grown up with.
And Pokemon is one of the best inventions man has made :P