2x4 blocks are what Lego is all about
My son got the Saturn V set (target age 14+) for his 6th birthday recently, which I was banned from 'helping' with. It took him a while, but he was able to build it. He now refuses to consider any sets which are not intended for at least age 8. He has an enormous lego collection, plus occasional access to my old Lego Technic sets, but he still likes building things out of standard 2x4 blocks. The specialised pieces are too specialised these day and no good for anything beyond that one function. Ebay comes in very handy for 'vintage' lego.