Hi there!

Here are my corrections to your sentences.  I hope this is helpful, let me know if you have any questions about them!

Medical scientists have discovered (for medical discoveries we usually use 'have' - present perfect tense -  when they are new discoveries which are being reported.  For old discoveries we use normal past tense. 'Isaac Newton discovered gravity.  Also 'discovered' is better than 'found out'. 'Found out is better for less important things or non academic/scientific things) found out that girls who are (comes from 'be physically active')physically active when they are teenagers generally live longer (this is the preferred way to say this phrase in my opinion.  I have read many articles about health and living longer, and they all seem to prefer this phrase). extend their life span. The research was conducted on seventy five thousand women aged from forty to seventy. It was discovered that those girls who were doing a lot of exercises ('exercises' in plural talks about individual exercises, such as push-ups or squats.  If you are talking about any physical activity in general, we just say 'exercise')during their adolescence suffered less from life threatening diseases such as cancer and a heart stroke. heart disease. (you may also want to say strokes here, heart disease and strokes are two different health problems)Even a short exercise of 15 minutes has a great and positive effect on their health.