No older than 75.
Practically speaking, it depends on whether you plan to do the physical work, what your plans are for the growth of the company, your financial resources, and so on.
I'd encourage anyone to do it, as long as you're aware of your limitations. If you're 18, go ahead and start a company. Know that you don't know everything, so keep your main job working for someone else, and start building a business based on small side jobs that don't interfere with your Monday-Friday work. The sooner you start to learn about taxes and the financial side of a business, the better.
If you're 50, go ahead and start a business. Know that you don't have the physical strength and stamina of a 20-year old, and hire appropriately. Use your life experience to connect with customers.
If you're 70, go ahead and start a company. You might not spend any time on the scaffolding, but you can be an owner's rep or CM on projects where you bring your lifetime of knowledge, and your ability to ignore all the BS that distracts the 18-year-olds.
What was the question again?