I want to do ironman when I turn 50. Dumb?

Awesome. Hopefully you keep yourself in outstanding physical shape. No - it is only dumb if you don't train smart ; stay in shape.
3 elements only. 1. Have a specific training program. 2. Create a sound nutrition plan. 3. Make the experience fun for you and your family. Keep up a good job!
Why wait ? I am 63 and plan on it by 65. At your age if you are experienced in tri and have the time to train -- why wait. I can assure you , from 13 years tri and 48 yrs running experience, that it is much harder as you age. If you think you are alone in your goal, just look at the 50 - 54 age group numbers for the Ironman events. Should have coach. Good Luck.
Brilliant. Plenty of us do it. I'm sixty and i swim circles around today's fat lazy ninetndo-playing spoiled bratty teens. Do it!