C is one month shy of 5 and has been creating his own masterpieces with K'nex for about 4 months now. Not sure some of the more complicated sets would work for a smaller one (A's got a ferris wheel that is quite complicated and C can't do it). But, the basic sets and most of the other sets are FINE for a younger kid.

What my kids love about K'nex is that unlike other building options, K'nex will bend and give more options for their imagination.