I do think we have a limit based on our body makeup, but we can push ourselves to improve. I think that certain training techniques work better for each of us, so it's finding what will increase your speed. I also think it's a function of your bike. I have a Trek 1200 and Lemond Zurich. When I was on the Trek I was averaging around 17mph. On the Lemond I average close to 20. I don't think I changed that much in my technique to justify the increase in speed. I guess also, how many hours a day/times per week do your friends train versus you? They may have more time in the saddle which could explain their increased speed.
"Only the meek get pinched, the bold survive"