Not everyone can whistle, but I have always been able to! I can even whistle and hum at the same time, as well as whistle by sucking in rather than blowing out. I like to think I whistle in tune.
When I was a kid, probably around 13 or 14, I thought I could compose tunes and so I whistled away with my little compositions. Of course nothing was memorable enough to survive, and so sadly I was no budding Lennon–McCartney.