You're never too old to wear braces but it's certainly best to have your teeth staightened in your early teens.
As a rule, orthodontics or braces begin when a child has lost all the primary teeth and the permanent teeth are well on their way into the mouth. However, there are circumstances which dictate early intervention. In other words, the child's bite, even at the early age of three or four, may be so bad that it needs immediate attention.
It is true however that you are never too old for braces. As a rule, no matter how old you are, something can be done to significantly improve your bite and appearance. And there are several ways to straighten teeth without showing very much metal. You don't have to have the full Tin Grin. There are clear ceramic braces, there are appliances that fit on the back side of the teeth, and there is a technique called Invisalign which is almost invisible. For more on Invisalign click their website address below.