If you are legally old enough to sign a contract, you're old enough to start a business and you're never too old to start. Most states require a person to be 18 years old in order to be legally responsible. You could start a business younger than 18 years old but you would need an adult to be responsible, sign contracts, collect money, obtain licensing, report taxes and establish banking relationships. Age doesn't matter when starting a business. What counts is your determination. focus and work ethic. The drive to succeed isn't dependent on how old you are.
What business and in what industry has some impact on how old you need to be to be successful. If you're offering services to a senior market, you will probably be better received if the seniors can identify with you. The opposite works as well. A younger market relates to younger entrepreneurs because the younger generations feels more in tune with their own.
Becoming a doctor requires eight years of college and medical school plus residency and sometimes additional education and practice to specialize. By default the medical and legal profession will be older when they start their practices or businesses. Attorneys may also have to apprentice at a law firm before they have enough experience to convince potential clients they can successfully represent them.
Some businesses such as landscaping, home maintenance and building require physical strength. As a person ages that strength and agility diminish. If you're starting one of these businesses, you'll be more productive if you can perform the work. Even if you hire employees to work, there will be times you have to fill in.
Some people have a first career that they've been educated and trained for and then decide after a number of years, it's not really what they want to do for the rest of their working lives. Since college majors are chosen at an early age, from 18 to 20 years old, it's not surprising that that choice doesn't interest someone more mature at 35 or 40 years old.
Starting and raising a family is a full-time job in and of itself. You may have more time and energy to devote to a business if you're young and aren't pressured by familial obligations. Or at the other end of the timeline, empty nesters who have seen their children off to their own lives have the time to begin a business.