If you open an additional American Express card account, such as the Starwood Preferred Guest credit card, it will have the same Member Since date as the JetBlue card -and- it will be reported to credit bureaus as if it had been opened in the same year. You can safely close the JetBlue card account, and you will probably be able to transfer some of the credit line to the SPG card. Alternately, you can ask about converting the JetBlue account to a free card such as the Everyday card, or one of the Blue cards.
^^^
I had no clue I could do that. I wonder if opening an SPG card makes fiscal sense period, but assuming it does...that's great to know.
Also, since my Citi AAdvantage card has only been open < 2 years and has a lower credit limit, I could probably stand to simply close that card and open the WN card...
My biggest concern is, as I said before, is the credit score. I'm looking to purchase a house in the next 1-2 years and in the grand scheme the credit score will be more important ($ wise) then the flight rewards.