This is certainly going to be on a kid-by-kid basis. Generally speaking, the best way to know whether or not your son is getting too old for Legos is to simply pay attention to how much he is playing with his current collection. It is important to note however that there are plenty of full-grown adults that still like collecting and playing with Lego bricks.  So, there certainly is no set cutoff for when Legos are no longer fun to play with.