①Ithinkthebeststageoflifeisyoungadultperiod,thatis,whenyou'retwentytofortyyearsold.②Inthisstage,you'retrulyindependentandfree.③Whenyoureachyourtwenties,youarejuststartingtoexperiencetheworldinitsfullextent.④Thisisthepointinlifewhenyoudon'thavetolistentowhatotherssay.⑤Itisalsoapointinlifewhenyouhavetimetoexperiment,andtryoutalldifferentthingswithouthavingtomakeapermanentdecision.⑥Soformyownpart,thebeststageoflifeisyoungadultperiod.上面这段话中第一句是主题句，第二句是主要扩展句。