I don't know if it's perks really - but make sure you get your Birthday Button. If you are staying at an onsite hotel, the front desk will be happy to give you one. There is a place in MK that will give you one (Guest Services? I'm not 100% on that).
Cast members go out of their way to wish you a happy birthday. I got a free dessert when I stopped for lunch. It's just a nice feeling. Some people mention that they get a lot more "freebies" while some mention that they get nothing more than birthday wishes.
I went for my 30th just this past January, and I totally loved it. I'm thinking of going again in 2014 for my next birthday. Nothing makes a birthday better than Disney!