YA Time travel/teleportation novel. Plot required using helmet (or other head gear)... but only for training. A young kid (less than sixteen?) finds he can either teleport or time travel (I don't exactly remember which one). He is given a helmet, and told that if he wears it, he will be able to travel wherever he thinks.
Later in the story, he is told he can actually do it whenever he wants, without requiring the helmet. The helmet was purely for training his mind.
The "gift" would eventually disappear as the person got older.
I remember reading this novel in my school library around eight years ago...