First of all, sincere congratulations on your Granddaughter's appointment to the Air Force Academy--that is a huge honor and I know the whole family is extremely proud.
Next, do not worry about Princess being "too old" for you. I think you will find that the length of cruise and the itinerary have more bearing on the age group and atmosphere of the ship than the cruise line. I've been to the Mexican Riviera many times on Princess and I can tell you they are very festive. There are sailaway parties from each port and at least one deck party at night that goes, well, past my bedtime.
One little tip: best place to be for sailaway from San Pedro is by the Terrace pool, all the way aft on Lido deck; you are near the Horizon Court buffet, the Outrigger bar, and you get a spectacular view of the L.A. and Long Beach harbors as you head South.
Have a wonderful cruise and stay in touch.