I've had one for 36 years now-- never had a lick of issues. Same here-- doctor's freak out and say "Lets schedule an echo" etc etc Each time the cardiologist just says I'm normal. Systolic murmurs are very common. You dont always outgrow them.
That's a good question to ask a doctor because they would know what type of murmur you have, what's causing it and whether it will progress over the years. I've had a heart murmur all my life (I'm almost 60 now) and it hasn't caused me any trouble. I love it when a new doctor listens to my heart, raises their eyebrows and tells me "You have a heart murmur." I just smile and say, "Yes, I do." Two of my kids have heart murmurs but there are no other problems going on and they're grown with children. So much depends on the type and degree of murmur. That's what you need to find out.