I consider myself succesful. I am 25 and have achieved a lot.
Reasons:
I am a hard worker. I currently work a full time job plus take almost 24/7 call (grossing somewhere in the low 6 figures.) I have a B.S. in computer science and I also am finishing up my Master's degree in Software Engineering. I started a small software/consulting business. On top of that, I am building my 5300 sq. ft. house.
I have a great family. Awesome wife and 2 little kids. My daughter (3) constantly makes me laugh, and tells me that she loves me for no reason at all. I am a good husband and awesome daddy.
I have no debt and spare cash. We manage money well. I wanted a truck, so we saved up (well, sold some investment property) and bought a used F350 with cash. I kept my wife's crappy car and it is my daily driver. We don't live paycheck to paycheck and we each have a 401K and IRA that we contribute to monthly.
I am able to provide for my family. The children aren't spoiled, but they don't go without. I am able to take my kids to cool places for vacation, which gives them a view of different lives.
I am happy. I work hard and benefit from my labor. When I drive to my house to work on it, I know that I have put in hundreds (if not quite yet thousands) of hours to make it possible. I know stuff wasn't handed to me. My parents helped pay for some of my tuition when I was young, but I also have had a job since I was 14, so I paid a lot for it (small U, about $3,000/semester.)
When I come to work, I know I am needed by many. I work an often thankless job, but knowing work wouldn't be smooth without me makes me not need recognition from others.
I am succesful because I am exactly where I wanted to be and then some. I have a wife who has been married to me since back when I was making $10/hour. I told her one day we would be making 6 figures, and a few years ago I proved myself right. Success cannot be measured by money, happiness, family, or any other metric alone. It is the combination of all of these.
If you are working a crappy job, have an okay family, and barely make it by, but "you are happy"....then you are happy, not successful.