You'd be surprised, though, sometimes. My 10 year old loved building the "age 16+" modular buildings, Tower Bridge, container ship, etc, when he was 4-6. Only grownup assistance needed was providing thumbs to mash things down occasionally, and sometimes suggestions on how to sort.

He'd still be doing it, too, if he'd not run out of room to build :P

("Age 16+" LEGOs really just require ... typical age 6-7 spatial skills plus not screaming in terror when you see thousands of pieces).