salamualaikoum sister,
First a warm welcome to you from turntoislam members as you are a newly registred member. I read what ya have written and i liked the questions you posed, becoz as muslim girls we need to get answers to them. As a muslim girl who reads about islam and who knows something about islam, it is my duty to enlight you with the small knowledge that i have in my mind about this fantastic religion.I will try to do my best to make you sister get convincing answers to your questions. Your first question was about the age in which a girl starts wearin Hijab, well, there is no narration from the prophet about a specified age for wearing Hijab, alla what the prophet peace be upon him said is that once the girl is grown up and starts having her menstrial period, at this stage she is no more concidered as a kid, she is a young lady who may attract men, so in this respect she needs to cover her beautiful body.
As for your second question sister, you said that some researchers claimed that its very important for human to be exposed to sunlight; my reply to this dear sister is that those researchers are not more knowledgeable than ALLAH swt and his messenger peace and blessing be upon him. I want you to always remember that Hijab is Farida, it is commanded by ALLAH, and as muslims we have nothing to do but obeying ALLAH's command and at the same time having trust in him, becoz ALLAH will never do something bad to his servants. He is the most knowledgeable and for sure if Hijab was going to harm us He the almighty would not command it.
the last point ya asked was about sports, my answer to ya is that islam did not forbid sports , on the contrary the Prophet peace be upon him encouraged muslims to practice sports. You think that wearing Hijab will not allow ya to practice ur favorite sport which iw raning, well my reply to this is that there is a narration in which it is said that our Prophet peace be upon him and his wife Aicha they used to run and compete each other, and that Aicha used to win , but later on when she got fat the Prophet won. so by this narration it clear that islam does not prohibit runin coz Aicha used to go with Prophet outside their house and practice their game. She used to do this wearing her hijab and there is no narration in which it is said that she lost because of her clothes. As for the tight clothes, ya can sear them under ur short jilbab inorder to absorb the sweat. the jilbab must at least reach your knees, i think if its like this it will help you run. Another important thing that you need sister to put into concideration is that if you run ya may attract man more than if you are walkin, always remember that men are watchin ya and watching your body and trying to discover its buty, so plz dont give anybody whom so ever he is this opportunity coz your body is a pearl that only one person on earth who has the right to touch it and discocer it beauty. My advice to you sister is to run just in front of women and if its not possible for ya ya can run in front of men but wearing your hijab , believe me it will not prevent ya from winning, becoz me too i'm veilled and i used to run with my veil and Hamdolilah i used to win.
wish you the best of luck
salam