The businesses that you start when you're 20-30, 30-40 or 40-50 in many ways reflect the amount of energy and the knowledge that you've gathered over the years.
Harland Sanders started Kentucky Fried Chicken when he was 61.
Sakichi Toyoda started Toyota when he was 59.
It's not really rare at all to start successful companies when you're older. Just go look at the history of big companies and you'll find plenty of examples.
When it wasn't, they said that he would curse enough to make a sailor blush, kicking pots and pans all around the kitchen, having a terrible fit. In really bad cases, he would take his "special equipment" back with him.
The reason he ended up going big was because he couldn't keep track of all the payments and legal paperwork by himself. The lawyer that ended up being a partner was told to go help clean out a desk -- and found it stuffed with checks totaling hundreds of thousands of dollars.
Not sure of the veracity of that story, but it makes for a colorful anecdote! And it reinforces the #1 quality of good startups: focus on delivering value to the customer and the rest will work itself out.
They started their "make it big" biz later in life, but they'd been in their biz for decades.
Too old would be 1 day before death, but you never know when that will be, so you might as well get started.