Well, I hope the waters warm.
years ago, I did an olympic. I wore a pair of running shorts, they were kind of brief and biked ran in those.
For transition, depends on the set up. On my olympic we just laid our bikes in an orderly fashion on the ground. The beach was sandy prior to the grassy transition area. I had a wash basin and a gallon of water with towel to clean off my feet. Kept my running shoes right there also.
Age, disadvantage. Probably your biggest disadvantage will just be lack of experience. I just did my first half iron man and I was quite suprised at what I learned. I imagine you will be as well.
With that said, I'd suggest just go into this as a learning/fun experience, worry about performing next time once you get your transitions, and training pieces worked out.
I wish you the best of luck.