SMH. He said "I'm not pure NOR am I a child." You guys misunderstood it.
I'll translate it so that you guys will understand. "I'm not innocent and I'm not a kid."
Now do you see how beneficial and important reading is, kids?
Hashirama thought he was pure because his eyes were normal looking when they were talking, so Sasuke activated his MS stating he wasn't pure, the eyes proving he had experienced loss and hate.