Hello, again. I apologize for not responding sooner. Got caught up in the holiday.
Background research. Think of your science project as an essay. You want to give an introduction and background information in the beginning. At the end of the experiment, if you find out there is a difference in memory capability between the groups, you will want to try and answer why that happened. After all, if you can't answer why, then why are you doing the experiment in the first place, right?!!?

Also, background research will help you ensure you are setting up your experiment with the highest probability of success. For example, say you don't find out until after you have completed the experiment that humans have no capability to memorize until the age of 12. Then, you wasted your time by studying the 0-10 age group. I'm not saying this is true...just an example of why you want to do background research!
Ideas for background research. This isn't an exhaustive list, but I hope it will give you some ideas.
- What is memory?
- Are there different kinds of memory? If so, what are they? What kind of memory are you studying with this experiment?
- How does the brain function to memorize?
- Does memory work differently at different ages? If so, how?
- Does memory deteriorate with age? If so, at what age? (Do your results reflect this? Perhaps this will help you develop your hypothesis)
- Are there different techniques humans use to memorize? Which techniques work best? (Do you want to ask this of your subjects?). For example, one might come up with an acronym of the first letters of each object to remember. Another person may be able to make a mental image of all of the objects.
- At what age do humans have the capability to memorize?
- Have similar experiments been done? What were the results? (May also help you to develop your hypothesis).
As far as your Rubik's Cube idea goes, I think it would also be an interesting experiment; and it is very doable. It will be studying a different kind of memory (background research question #2 above!), so be sure what you will be studying with the experiment is, in fact, what you want to study. Suggestion: a simple Google search of "different types of memory" will help here. Is a 5 year old capable of learning a Rubik's Cube sequence?
If you want to stick with the Rubik's Cube test, take a look at this experiment. A different angle, but it might interest you.
http://www.sciencebuddies.org/science-fair-projects/project_ideas/Math_p023.shtml#backgroundI hope I have answered your question. Feel free to continue asking questions. This should be a great experiment!