I was young, probably 7. many things were SOOO obvious to me
1. big signs in stores saying "Great Stocking Stuffers!"
2. It is waaay too cold for someone to fly around the world in december
3. How did Santa get ALL those presents into one sleigh
4. Reindeer don't fly
5. How does anyone have a workshop where it is THAT cold? (I was under the impresion the poles were like -50 F all the time)
6. How can he get arround the world in one night (once I think I even calculated it with time zones, I was like 10 I think)
7. My parents used wrapping paper from Santa one year, then it was in the wrapping paper bin 2 years later and on presents from them 3 years later
8. Santa's fancy handwiting changed styles every year
9. The really poor kids don't get presents so we send presents to Toys for Tots
10. Down the chimeny? Come on, we lived in an appartment, and I remember telling my parents one year when I was about 5 to leave a large window open for Santa
11. Santa is at EVERY mall every day! My mom told me he sent his friends to tell him what kids wanted (I asked why they pretended they were really him)

I am by no means trying to bash on Christmas, but these are the things that went through my head every Christmas.
As for presents from Santa now, My 3 year old sister belives so my 8 year old sister and I still get gifts signed from him.
I never let my parents buy the wrapping paper or do any of the stockings in front of me though, I want to be surprised.