An often heard phrase from a child is "I can't wait until I grow up, so that I can ______" here you can fill in the blank with a few of the more common ones such as: stay up late, go to school, not eat vegetables or feel free to choose one of your own. And no matter how many times you tell them not to be in a hurry, that being an adult is not as easy as it sounds, they still look at you with a disbelieving expression. Probably very similar to the ones you used to give to your parents when they tried to impart the same wisdom on you.
While you do want them to slow down and enjoy their youth, you also need to start a little preliminary "facing adulthood" groundwork. Many experts feel that having chores to do is one step towards the ultimate goal of raising a responsible adult. Of course, they need to be age appropriate and in-line with their own personal abilities, otherwise, frustration sets in and the child’s self-esteem drops. Keep in mind that most children only focus on their immediate emotion, so telling them they will be sorry later if they do not do this now or one day soon they will be glad they did it, is not something that is going to make an impression. Instead, work on setting short term goals for completing their chores and once they accomplish this be sure to praise them for a job well done. If you would like to include some type of a reward for doing so well consider something that ties in with them growing up.
One idea is to get them a Kids Duffel Bag, allowing them to replace that small "baby suitcase" with Going to Grandma's written on it. Not that there is anything wrong with going to Grandma's mind you, but imagine their smile when Grandma tells them how grown up they are getting to be the next time they visit. Besides, a teddy bear probably fits better in a duffel bag.