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The information is too limited and the context of the situation is also not clear,
This can be a perfectly normal thing, but is unusual
since generally speaking the peer groups, likes & dislikes and the developmental stages are vastly different in these age groups
I would recommend that if there is any concern, then this may be gently evaluated by a child counselor/ psychologist, so that the exact nature / reason of this friendship can be elucidated
Please get back for any questions,
do leave a positive feedback too