'I got my nailgun'

Drew has become Dad's little helper around the house. When Scott mows the lawn, Drew gets out his little mower and follows him around the yard. The other day I asked Scott to take a look at the dryer. It was making a clunking noise. Scott started looking at it, and Drew disappeared downstairs. He came up with his toolbox and his nailgun saying, "I can help. I got my nailgun!" Too cute!
Why is when they are 3, they are happy to help, but when they become teenagers, or for that matter 5-year-olds, it's "ahhh. you want me to do what!"