Some women like to be bitten pretty hard, but only after they are "warmed up". Just saying, you should probably ask them before doing this.
I've had several gfs that liked this. They kept saying "Bite harder. I said HARDER you prick!". I said "Really?" They said "Yes!"
As far as younger girls go (teens and twenties) they are still growing and their breasts are often sore from raging hormones. So be careful.
Don't focus too much on one spot. Go for 2-3 minutes per spot, then move to another spot. They hate to get sore, and so do I.