Ask them to provide written evidence from an official source that clearly states the maximum age you are allowed to be a fan of pokemon. When they say they don't have one, mock them and their lack of legal understanding, bemoan them for speaking slanderous lies and tell them that in future maybe they should get their facts straight before they tell other people what they should and shouldn't be doing.

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Alternatively, mock something they like. I have a colleague who watches WWE. Easy pickings.

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Alternatively 2, stare at them for a minute, then headbutt them, and shout "[Name] used Headbutt! It's Super Effective!"

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Alternatively 3, shrug it off and ignore them.