Skittish about opening credit cards

Oh, how I wish I were not so paranoid about my credit score! I really want to apply for the Citi cards that have the $100 gift card, but I'm worried that doing so will hurt my credit score. Not that I actually need to get a loan on anything anytime soon, but since I'm young and don't have a huge credit history, I'm skittish about doing anything that will lower the average age of my accounts. On one hand, I've got excellent credit so far, and I don't need to use my good credit score for anything in the near future. (Although my landlord did pull my and Boyfriend's credit reports when we applied, I'm not entirely sure why. But we both had scores over 700 so I guess we passed.) On the other hand, you can never tell what you might need to do tomorrow that you don't know about today. Maybe opening a new credit card won't matter.. I'm curious to find out, for $100!