I'm a 23 year old developer working for a medium size startup. I made the switch to my current employer about four months ago from a corporate job. While I enjoyed the move for the first three months or so, I do not feel the same now.
The reason is that recently one of "star" developers at my current employer was poached by another company. As a result, most of his work (which I had no clue of) came on my shoulders. The code that I inherited is unidiomatic and written in a way that only the author could understand. And now, since I "own" the code, I've been putting extra hours and working on it as much as 15 hours a day to understand it and make fixes (which has become my day job). Despite of all this, I've failed to deliver on most deadlines (as the managers had higher expectations from me). To add to all of this, I've been diagnosed with hypertension, that gets worse with stress.
Given this, should I risk being labelled incompetent, by telling my stakeholders about my problems or should I put in the long hours needed to get stuff done (as that's what people in startups do).
Gaaaaaaaaaaah.
Sorry, that was just my scream of frustration about a world which doesn't make this answer obvious.
You're an adult now. Go to manager; say that you've inherited a code base written by a more senior developer which has a substantial amount of technical debt in it and you'll need time to get up to speed on it. Work together to get a better understanding of when the milestones will be shipped successfully. (Unobvious-to-younguns true fact about managing: managers often need predictability a lot more than they need speed.)
Do not work 15 hour days to try to grok the code better. If you model your manager as a heartless automaton (and he isn't!), he'd heartlessly prefer you to be resting at home rather than in the hospital, since only one of those means you'll be filing TPS reports tomorrow.
Do not attempt to sprint your way out a scheduling issue. The business will always be capable of finding another catastrophe. You know how there are a herd of cows or a murder of crows? There's a startup of catastrophes. Companies/individuals/teams which develop crunch cultures never stop crunching. They ship whatever the blocked milestone was -- late, of course, because crunching very rarely actually speeds shipping -- and then promptly identify a new milestone to crunch towards.
It is not the job of 23 year old developers to insulate the business from the predictable consequences of the actions of more senior employees. If your manager feels differently, there are many saner managers available. That's a pretty radical step, but choosing burnout is a much worse step. Burnout is not fun. You wouldn't choose to develop adult-onset diabetes because it would avoid a difficult conversation at work. Don't choose burnout, either.