Yeah, they will have shorter lifespans. anything putting stress on the body will reduce your lifespan(testosterone and estrogen both reduce lifespans. being a eunuch is the most stable in terms of longevity). But really, if you're young and you transition, try not to think ahead to when you're going to be 50 or 60. Rather, think of it this way... 50 or 60 was considered beyond elderly at some point in human history. It's plenty of time to live a full life. some people live full lives by the time they are 20(or younger). Some of us may be outliers in terms of age, some will die young, some will reach 120 with medical advances that will inevitably come along. The best thing you can do is try to be productive and chug along. Do something interesting in your 20's and you're thirties so that when you're long gone and 200 years have passed, people remember those years about you.