It doesn't matter. 1 piece 2 piece Whatever is comfortable. Don't let someone tell you what's in appropriate and what's not. If you think your daughter should wear a 1 piece make her wear it. I let my daughter wear 1 or 2, but she generally makes her own decision to wear a 2 piece with a shirt on top, and sometimes even shorts. But obviously some swim suits are to revealing for girls who are so young so just be careful what you buy.
maybe i am old or old-fashioned, but i hate to see young ladies in a bikini. not only do i think it is extremely dangerous, but i think that by allowing young girls to wear bikinis (as well as other clothing styles too mature for the age) we are pushing them too quickly into sexual awareness and maturity that they are too young and naive to handle responsibly. television and other forms of media are already forcing sexuality on our younger children, we need to slow that down a bit with guidelines. - my daughter was not allowed to wear 2 piece bathing suits until she was 15, and not allowed bikinis until after she left my home. my 21 year year old daughter now thanks me and understands, but at the time i was "ruining her life!"
try something that covers your middle. I used to wear one pieces all the time, but then my mom said I could get a 2 piece on one condition: that in the end result, my middle was covered. I ended up getting a tankini.
Totally depends on the child's parents? They make those decisions, not the 13 year old. That being said, 13 year olds can be mentally or physically more like a small child or a high school kid. Level of responsibility is a factor, but the pubescent doesn't make these decisions, adults do. And why is this 13 year old trying to entice a sexual response? Better question.
Well whatever your parents think. It is not up to the thirteen year old. If your parents say you can wear a bikini but your uncomfortable in it then dont wear a bikini. Over all its the parents and kids decision.