My eldest, BP, is 9 years old and has never really been that bothered about Lego. We've bought various kits over the years and he's quite happy to make whatever the thing is but then quickly loses interest after it's finished. He hasn't ever used Lego to create his own things, made up dens or buildings using the Lego. I decided a while ago not to bother buying any more kits, Lego is quite expensive after all.
A few months ago we were wandering in the toy aisle at Tesco and happened to come across something called Lego Mixels. Small Lego kits that when pieced together make little creatures. BP wanted one immediately and decided to use his own money. He made the first one in about 20 minutes and wanted to go straight back to Tesco to buy the rest.