Most kids my age tell me that it's wrong to say 'Merry Christmas' so I should say 'Happy Holidays' instead.
Most adults say 'Merry Christmas,' in fact a poll taken in one state of America said only 2% of people like 'Happy Holidays' and 98% like 'Merry Christmas.' This poll had many people of many different faiths.
I can understand where the kids come from though: In school we learned to sing "We Wish You a Swinging Holiday," made 'winter trees,' and one year in middle school when a Jewish friend of mine brought in a dreidel so he could teach us how to play at lunch, he got scolded and it got taken away. I'm not going to agree or disagree with any of this just to keep the point of this status clear.
So, this year I decided to split the difference between the ages: I say 'Happy Christmas.' :)