Don't worry... I was 5 yrs. old when they found mine. The cardiologist called it an "innocent" murmur. I am 26 years old now and occasionally a doctor will say they can hear it, but not that often! Mine was discovered after my brother was born with pulmonary valve stinosis. He went in for heart surgery at 10 days old, and will continue to need surgeries his entire life. So, that is why my docs wanted to get my heart checked out. Many murmurs correct themselves. So, don't stress... wait to see what the cardiologist says. Most likely they will do an ultrasound and/or MRI to see what is going on in there. Keep us posted!
They can definetely develop in later stages of growth. I know for a fact that my little bro had a heart murmur found at 5 months (his was a small hole in his heart), and it gradually over time closed up. I had one when I was younger, and by the time I was 14 I no longer had to see a cardiologist because mine had gone away. Low and behold, when I found out I was pregnant, my OB told me I had a heart murmur. I mentioned that I had used to have one and she said that most pregnant women who have had one prior will get it back during pregnancy, and after pregnancy it will most likely go away...weird!
A heart murmur can be something that is developed, and or born with but not noticeable enough to catch until later on in life. A Heart Murmur is something that generally fixes itself as time goes by. My husband had one at 4 months of age, and it closed by the time he was 2 years old, and hasn't had any complications from it.
I wouldn't worry...I think they can develop...lots of people have them and it is probably just precautionary to send you to the cardiologist.