Warren Buffett made the bulk of his wealth after his 70th birthday. Even though he started investing and building businesses when he was a teenager, it's the last 20 years that have really counted.
Buffett has benefitted from the miracle of compound interest. All the hard work, the little wins have compounded year after year to give today's result. And it's the same with all of the billionaires in the ranks of the richest people in the world. It takes time and patience to build wealth and let compounding do its work over the long run.