I'm wondering how they figure out what the children really thinks. Having a birthday party, as well as defining age in terms of years, are both defined by human

I'm wondering how they figure out what the children really thinks. Having a birthday party, as well as defining age in terms of years, are both defined by human beings. With that logic and lack of background of the weight of the "cultural importance" in each of birthday party and definition of years, I don't find it difficult to believe a child could think a 40 year old could define herself as a 29 year old if other people would hold that birthday party for her. What I'm saying is - are the children aware our definition of age in terms of years is linked to the physical state of the human body?