You are likely to get some altitude sickness but mild pain killers and anti sickness drugs usually keep it under control. The use of diamox is controversial - I took it for the summit bid.
I have heard of altitude sickness but was not familiar with the symptoms. From an article I read it seems very common. 1/4 of vacationing Colarado skiers develop it as well as 2/3 of Mount Ranier climbers. I had a friend develop a mild form of it while in the Andes. He had to descent to a lower altitude.
I always wish that I could have had an adventure like this. Now I'm over the hill and just to old Have a great time!
Who is telling you that you are too old? The last time I climbed Pike's Peak (14,110), my hiking buddy was a 67-year-old man who had been a smoker for over forty years. The final two thousand feet of elevation gain were a significant challenge for him. We had to make frequent stops to catch our breath, and for the last thousand feet I never let him out of my sight for a second in case he collapsed. But you know what? He made it all the way up.
I know I'm just some stranger on the Internet, but I would suggest you change your thinking from "I'm getting too old to have an adventure" to "I'm getting too old NOT to have an adventure". You'll never again have as much life left as you have right this second, and that will be true the next second, and the next, and the next. It might take you longer to get into endurance shape, and you might not be able to ascend Kilimanjaro, but there are hundreds of smaller peaks right here in the United States. Don't give up on your own personal adventure whatever form it may take. It's never too late until you're in the ground.