MY GIRLFRIEND IS A SLUT?

my girlfriend and i have been dating for only just over a year. for your information, i'm 20 years old and my girlfriend is 19 years old. we are both currently virgins however my girlfriend keeps asking me to have sex with her even though we are both too young for that and only been together for 1 year. how do i explain to her better that we are both too young to have sex and that being together for one year is not long enough to have sex?