I'm 33 and still believe in Santa.
I made a covenant / contracted with the spirit of christmas when my son was born to, as best as I am able, stand in. Each and every year, though, that spirit is very much around. Whether it's a bumper year, and I'm able to help others keep their covenant, or it's been a rough year, and someone else is helping me keep mine.
When I was 11 years old, I caught Rabbi Moskaweicz with the spirit of santa... as he snuck around the navy base delivering presents on the window sills of kids whose dads (or moms) were deployed over the holidays. It never occured to me that that negates Santa. Instead, it's one more proof. One more kind act. One more twinkle in the eye, and finger on the side of a nose. One more piece of magic.
I've seen hundreds and hundreds of people you might never suspect with the spirit of christmas.
And those are only the ones that I know of.
Call him what you will; Santa, Father Christmas, The Holy Ghost, Saint Nicolas... whomever the Spirit of Christmas happens to be... I care not.
Santa's real.
If you don't believe, that's fine. We're allowed to hold different beliefs. <grin> Whether or not God exists, depends on who you talk to! People believe differently.
SO... My son is 10. He knows a lot of people don't believe in Santa. He has friends / family in nearly every major religion, though... so he's used to differing belief systems. It doesn't phase him