My erection question

I get erections quite easily and then they go away within 10 seconds if I want them to. Sometimes they are quite random. I am also 19.
Sometimes when i masturbate without any arousing images i will lose my erection for roughly 10 minutes without ejaculating.
Also, when i am not erect, my penis size is a 1/4 of its erect size.
Is this fairly normal or what?