Shopping by age is a great place to start. Click on the age you're buying for from the drop down menu, which will show you everything I'm suggesting is suitable. To narrow down your search click on the 'refine' button, select a category and then hit 'apply'. No need to click an age as that's already been applied.
I personally recommend buying toys that are right for the age despite parents thinking little Julian is a child genius. Toys have been designed with an enormous amount of thought and kids really get the most play value out of them if they can use them correctly. Saying that, my 4-year-old nephew is like rain man and can happily build a LEGO model targeted at 12-year-olds. I suspect he is an exception though. He gets it from me.
Don't want to use the drop down menu? Just click on the age you're shopping for from the list below and it'll take you straight to some great gift options.