You are not too ambitious, but you should remember that it takes a long time and a lot of work, a few failures, and a hell of a lot of risk taking to get there. If you look at the most successful people, they share three dominant qualities.
1) They are great observers of people and the market around them. They were able to find a niche because they studied people’s behavior and market inefficiencies. This is the biggest obstacle to young entrepreneurs. By the time you have the experience to define an opportunity and the experience to execute it, you are many times too burdened with responsibilities (house, spouse, children, aging parents to care for, etc.) that you are less likely to take the risk to try a run at it.
2) They are willing to take risks. You hear about successful people who have failed multiple times before hitting it big. I would argue that while they certainly learned a lot from a failure, it’s their willingness to take a risk again, even after failing that made them succeed.
3) Finally, they are disciplined. All of the wealth, success, and fame can be destroyed by a lack of discipline. Over-spending, drugs, gambling, reckless or unethical behavior, and laziness, have been the root at the downfall of many otherwise successful people.
So go for it. Think big. But, remember that you position yourself in your 20s and 30s, to be successful later in life. Don’t get discouraged in the beginning and be realistic in your expectations.