Link-baitey title. Even though the content says any age is fine, the title assumes that there must be a certain age when one is too old to start a business.
Here's the real answer: about 1 nanosecond after you've died.
Seriously, don't let anyone tell you that you are too old for anything.
It depends on the business and on one's financial capabilities. The most important thing is "are you doing something that makes you feel happy about it?" In theory one is never too old to start a business since a business should be a journey, not a destination.