Discretization of a continuous variable

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He guys,

I'm analyzing a research regarding patients and their risk factors for developing a certain side effect.
If i enter age as a continous variable into forward stepwise logistic regression - sex and the diagnosis of the patient have significant OR's , age is not included.
However, if i say, i believe children will have more side effects, so i discretize the age variable into <16 and >16 years nominal variable called kids. when instering this variable instead of age into the stepwise logistic regression it's significant and have an amazing OR.

How can this be? I could have chosen any random age, and try it out...
What is the real result?

A detailed explanation of this difference will be very appreciated.
 
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