From what I have studied there is definately no age limit but don't scare yourself into one either. Look into family history if you can and demand more testing and find out what is going on. Take a record of what you are doing when it happens.
In another thread I wrote that a friend of mine found she had something similar to a heart murmur but it was slightly different. It started when she was born supposedly but just started showing it's ugly head (probably from age) and she found out what it was by asking for a monitor that she was to wear for a week and keep a record of her activities. Now that she knows what it is she just has to be careful and work with it.
Don't worry, just keep on it and ask for a specialist's opinion. And yes sometimes warning signs can go on long before something serious happens because the problem could be building up to something and this might take time before it is very serious. Many have warnings but think it is gas or stress, even a pulled muscle in the chest. When I was a WCB First Aider I had a guy lay down on the job with tears in his eyes scared silly. I felt his chest and told him he was ok. Turned out one side of his chest was hard as a rock and the other side was not. He had a regular pulse (slightly raised of course because of his fear) and what had happened is he went to jump on his forklift and pulled his shest muscle because he made the move so suddenly that it would not let go. He went to the hospital of course where they verified my story and sent him home for the rest of the day. This muscle bothered him for days after but he was ok and still is doing fine. Part of my theory might also be that he is a very stressed out kind of guy and that might have added to his event. Now he is doing a whole different type of work and feels much better.