It's not just impulse control. That poor adult probably needs the "marshmallow" now while the wealthy adult has enough resources to "invest the marshmallow". There is also perceived risk. If the marshmallow is there for the taking, you are guaranteed to receive that marshmallow if you take it now. The promise of receiving more marshmallows now is just that, a promise not a guarantee. (And, of course, many real life future gains are not even promises. Many people invest a lot into their homes with the expectation that it will increase in value. For most it will. For some it won't.)

It may sound silly when everything is framed in terms of marshmallows, but it's probably a safe bet that lessons learned in life will carry over to an experiment unless they carefully consider what the experiment is asking of them.