Is it normal to still believe in santa claus?

I'm asking this on behalf of my cousin. He's 23 years old and he still thinks Santa Claus is real.
He's a smart individual, he's not retarded or anything like that, he actually has a good job and a college education. But he still believes in Santa.
He still puts out milk and cookies on Christmas Eve, and when he sees the next morning that they weren't eaten, he says "maybe he wasn't hungry, but he's real damn it." He still thinks that he has to be asleep or else he won't come, and one year when we had a party on christmas eve, he was telling everyone around 10:40ish "i have to get home! santa's coming!"
He actually writes his Christmas lists to send to Santa. Believe it or not, he even talks to the mall santas about what he wants. I remember being with him when he did it, and everyone was giggling, and the santa actually asked him "are you kidding?" and my cuz was so serious and said "no! that's what i want!" He then tells me "i know none of them are the real santa, but they all work for him"
One time we all tried to convince him that he wasn't real. He refused to believe it and he got so angry he drop kicked the coffee table and shattered it, and then stormed off to the kitchen crying.
I highly doubt he's making all this up for a stunt or a joke, because of how genuinely serious he is about it. We all know Santa Claus is just a childhood myth. Is this normal? It's pretty funny, but at the same time I'm kind of worried about him.