If you have some idea that pops up, the early twenties would be a good age to start a business. But as I have mentioned in my earlier reply there is no age restrictions and I have given examples of successful businesses with no age barrier.
Early twenties has a few advantages
1. You are young and active and physically fit to work harder.
2. Since you are fresh out of your education, you are full of enthusiasm. Also, young people with good business acumen are good at making networks.
3. You can take risks. That's an age when you do not have many liabilities like children and family etc. to hold you back from taking some big risks. And being a fresh youth you will not end up in lot of analysis paralysis. You can take chances on fresh ideas.
However, being so young you might lack experience, will have no or less savings, and end up taking huge loans. But then you have a longer time to set your business and pay them back.