By "starts at 10-11", what they mean is that age is where the physical processes take their very beginning - it's not a useful measure for laypeople to use. A 10-11-year-old will typically not have developed breasts or started menstruating (unless they are VERY early bloomers, likely due to some underlying condition), and that very beginning of puberty will not have immediate overnight mental effects. 14-15 sounds like an absolutely reasonable, plausible age for a girl to start thinking about sex. Prior to that, they may have started to take an interest in the opposite (or same) sex, but that's more likely to be in swooning and kissing than in anything more intimate. Even once the body hits fertile age (menstruation starts, for girls), it's not necessarily a good idea to immediately jump into bed with someone. If you look at practically all reputable breeders of pet animals and livestock, they will not immediately breed their animals the moment they hit fertile age (ferrets are an exception because their biology is messed up), but rather wait until they are closer to being done growing, in order to avoid adverse effects on the mother. While that's focused on reproduction, while you're presumably mostly advocating for recreational sex, we do need to acknowledge that even using condoms or other forms of birth control, there's practically always a small outside chance of birth control failing. And the younger and closer to menarche a girl is, the less likely she is to notice the irregularity in her periods that's usually the first telltale sign one's caught pregnant. Can you see where I'm going with this?