Startups can be opened at any age. Bill Gates and Mark Zuckerberg started at a very young age while Henry Ford started Ford Motor at a relatively mature age. There is no right or wrong age for starting a startup.
Generally, I see that it is easiest to start a startup in college. In college, parents do not push an entrepreneur to support the family. In fact, parents are happy to support the entrepreneur while he or she is still in college. These days many colleges have e-cells and incubators to support student startups. Many times professors and friends are also supportive during college days. The best thing about starting a startup in college is that if the startup does not work in the first few months you can always find a job after completing your college.
After college, you can start a startup but it becomes a little more difficult as generally the entrepreneur has peer pressure and family responsibilities. I would advise that entrepreneurs should do 1-2 jobs after college before starting a startup.
Another good age for starting a startup is around 40. You are more experienced at that time and have good connections. Plus you get time to save money for the bootstrapping. Thus 40 is a good age for starting a startup.
I had an uncle who was very entrepreneurial and wanted to start a startup in his 70s. Thus being an entrepreneur is more about passion and mindset than about your physical age.
You should open a startup when you feel you have found a big problem which you can solve. You have some money to bootstrap your startup for at least 1 year and you feel that you cannot live without starting your startup. Open a startup when your passion for starting a startup is running high.
This is an extract from Saurabh Jain's book - Startup Canvas, available on Amazon