"You're a grown adult! You can do anything you want to."

I never understood comments like this.

It reminds me of a discussion, I had with a former paramedic. She told me, "It's not a job for everyone." What? There is no one job that is suitable for everybody. Comments like my example and your quote are generalizations, at least that is how I interpret it! Anyway, to accomplish "anything" one must believe they can.