You can learn to play the violin at any age. I remember seeing a four year old playing violin like a master before. Forget about ever hearing the phrase, "You can't teach old dogs new tricks," as well. Even if you don't, stop thinking of yourself as a dog! You can have anything you want if you're willing to go for it.
Some qualities will make a huge difference in your success though. These qualities can be learned, although for some it is harder than others, but they have to be there in order for you to excel in playing violin.
You need to want it enough. If your drive isn't high enough you'll give up at the first obstacle. If there weren't any obstacles, it wouldn't be worth doing though. Sure, you can improve or supplement your drive with other techniques, but it has to be present somewhere in you.
You also need to be willing to take regular action towards mastery of your instrument. If you aren't practicing regularly, you won't see improvement. Scheduling this practice time works wonderfully. The longer you maintain a schedule when it comes to practicing your violin lessons, the easier it is to maintain. This will be difficult at first if you haven't done this often, but stick with it and you'll see wonderful results. When it is a habit, you won't be stopped by times of low motivation or depression. It just won't seem right not to do it.
Creativity is needed to supplement discipline, should you be weak in that department. Get leverage on yourself to ensure that you practice no matter what. It has to be strong enough leverage to get you through the times that you feel bad, are tired, or don't see the point. It can't just be about mastering or not mastering the violin.
Should you value your honesty a lot, tell everyone that you'll be able to reach a certain goal by a given time, and invite everyone you can to watch you do it on that date. If you're in rough financial waters, bet your last few dollars to someone that you'll be able to play a given song without error by whatever date you choose. If you can find a way to back out of it at the last minute, it isn't leverage.
Generally, those who have taught themselves violin are more disciplined and motivated. Get in contact with some of these individuals if at all possible. Ask them lots of questions about what they do and why they do it. Another good thing to do is watch live performances from violinists who make your jaw drop. If you can make the violin reach other areas of your life, it will give you a greater sense of purpose when you practice.
Think of this as a commandment for those who wish to excel: Love thy violin. Take care of it so you're happy with it. Don't neglect it, as that will ruin the value and your pride in your playing.
If you maintain these efforts, your progress will be remarkable in time.