What are genetic diseases that shortens lifespan to 18-25 years? how is this being treated?

Difficult to say: There are likely hundreds of genetic disorders which might kill one individual in that agegroup, while others with the same mutation may survive. One of the most frequent, however, is Duchenne Muscular Dystrophy, and a later-onset Spinal Muscular Atrophy. Kids who rebel against the necessary pulmonary care to keep them alive, in cystic fibrosis, die early, instead of getting to their 30's.