ANDY'S ANSWER: Well, I know of relationships that started out at your ages that have lasted for over 50 years. But I also know a lot more relationships that that didn't work out more than a few years, both for people of your ages and those who are a lot older. My point is that your age is not necessarily the most important factor to determine if someone will be able to maintain a lifelong relationship to another.
I think that what most people don't realize is that love alone WILL NOT carry you through the rough spots in life with your chosen partner. As a matter of fact, even in the best of relationships, there are times when people just don't feel the love at all. Many people's relationships fall apart once the passion has abated. There is an unrealistic expectation of a fairy-tale "and they lived happily ever after". In fact, a successful relationship takes a lot of work, and sometimes it can be extremely difficult. But every time you are able to work together over those rough spots to get to the other side, it adds another thread to your bond.
From what I've observed, the best relationships are those where each person considers their partner in every decision they make. They think of themselves as a team at all times. They have made their commitment to each other with the understanding that failure of their partnership is not an option. They also understand that there will be times that one partner appears to be doing more than the other. The important thing is for each partner to believe at all times that the other is doing the best that they can. There will be times that one has to do the pulling for the both of you.
I think that most important of all is for you to like and trust the kind of people you are, and mutually agree on the direction you want to go. Discuss everything that you are thinking of for the future. I can't tell you how many relationships have fallen apart because people discovered something about the other that they couldn't live with. If it was that important, you'd think they'd have talked about it ahead of time, wouldn't you? But that just goes to show that the unrealistic notion that love conquers all is the real fairy tale. The reality is that each of you investing commitment, trust, understanding, working together, compromise, and being willing to put the success of your relationship above your own wants every single day of your lives will be the factors that determine whether or not it will last.