Sometimes i can picture growing old with my bf and sometimes i cant, is this a problem?

At age 20, it's not a problem. It's pretty hard to picture life 30 years hence when you haven't figured out where you'll be 5 years down the road. Relax and enjoy right now. Maybe you'll still be with the same guy and maybe you won't. The time to worry is when you can't see yourself with the same guy by next weekend, then it's time to move on....