I just got a new credit card from Citibank (double cash back card). Whenever the Citibank Visa for Costco is available, I would like to get that too. When I get the Costco card will this lower my average account age?
The new card (Costco) will lag the old card by only these few months, but, you now have two cards that are newborn, and start the clock on both. If you have 4 cards with a 10 years average, you're now at 6-2/3 years. Worse, If you just had 1 10 year card, no others, you're at 3-1/3 years average after getting these 2 new cards.
In the big picture, this will only impact by a small number of FICO points, but if you are a deal chaser, you can have a card age that never gets to the 10 or so years to maximize your score.