The two pictures are when I was 13 years old and I’m and 18 girl. My hairline shape is changing. Should I be worried about this?

The 18 year old picture needs better quality, but it does look like the shape is changing. This is not uncommon. I studied children from the age of 5 (both boys and girls) and saw rounded hairlines if preadolescent children change in both boys and girls as they got well into their teens.  The rounded hairline when you were 13, has flattened out now and that is very common and normal. If you think that you are losing hair, then see a hair doctor.

juvenile hairline2evolved hairline 18