I've got a 2.5 yr old and an 11 yr old. The toddler loves his Duplo's, but doesn't build specific things with them. Just randomly mushes them together.

My daughter who is 11 now, grew out of that phase at about 5. She wanted real Legos and wanted to follow the instructions exactly to build what was on the box. Maybe that was unusual, she is very uptight, but its what I was basing my experience on.