I wouldn't let her wear it.
Answer: Regarding the shirt, if she were my daughter or my little sister, I would not let her wear it. I agree with you. It is suggestive.
Regarding the age question, that is a lot more complicated. At the same age, girls are much more mature than boys, both physically and emotionally. This means that younger girls, say 15 or 16, are dating (and having sex) with older boys, say 19 or 20. In many states, such an arrangement could result in the boy going to jail for statutory rape. You say your age group started at around 14 and 15. Were you having sex with boys who were 14 and 15? Not likely. You were probably having sex with boys who were three to six or seven years older. Sex, as you obviously know, is a wonderful and fun thing to do. But it can be full of danger. Pregnancy is a possibility. The transmission of diseases can always happen. And if both partners are not in full agreement about the relationship, it can cause emotional problems. While a 12-year-old girl may be physically able to engage in sexual activities, can she be prepared socially, emotionally and psychologically for such activity? How will she feel about herself the next day? How will those feelings influence her future behaviors? Everyone is different, both intellectually and physically. There is no right answer to your question about an exact age. It will vary from individual to individual. Some people still think that both the female and the male should wait until they are married to engage in sexual activities. However, while that may be an ideal for those people, it is not realistic. When those hormones begin raging, activity will begin. Just because there is some law on the books or because some church doctrine says to wait, people, being people, will do what their hormones tell them to do. As long as force is not involved, I think that there needs to be a relaxation of some of the "sex laws." If force is involved, then I think the "sex laws" should be even stronger than they are now. I could still bring up another dozen or so issues that your question has prompted. I can only answer by saying that the "right" time will vary according to many factors. Age is only one of them.