5 years is nothing, as long as you love each other, just keep it low profile, don't be too obvious, don't scream in the streets that you are dating etc. eventually, if the relationship continues when she is 24, and you are 19, it'll make no difference at all. the person i loved the most in my life was actually nine years older then me, and i started dating him when i was 16, we ended the relationship because we both went separate ways in our careers heh :) thats life, gotta move on all the time.
if it doesn't work, then thats ok, just be happy knowing that when you did love her, you were with her, and that you kissed her, create wonderful memories together :)