Some commenters are taking context into account, but let's talk about it directly: what are your career options?
I see you're in NYC, which hosts many companies of different sizes, structures, norms, and projects. Woot! That area is relatively amenable to junior designers, and if you have your pick of several career options, I'd recommend an early jr. Designer run with a bigger company (more instruction, clearer paths to grow) and more mature jr. designer take on the smaller company/solo designer for a start-up route (forces you strengthen your development skills, which you'll be in a better position to do if you really have your fundamentals down. Not perfect, but down).
However... if this start-up is your only option, or were you in Little Rock, AR where there are few opportunities open to jr. designers, I'd recommend you take this position on the condition that a thorough investigation of the company and its people raises no red flags. If the choice you face is working at a start-up known to destroy designers versus working at a low-paying job and having freelance stuff on the side to up your skills, take the second option (your skills are more likely to improve, and you're less likely to face serious health issues).
Last note - on mentors: Like others have said, find several mentors, not just one. Recognize the relationships you have with them are largely dependent on your interest in growth and ability to organize your communicable goals, though they're even more dependent on how much time each mentor has to spare. If you can, ask people who have 1 or 2 years more experience than you for mentorship since they'll be more intimately familiar with the problems you're facing.
Some commenters are taking context into account, but let's talk about it directly: what are your career options?
I see you're in NYC, which hosts many companies of different sizes, structures, norms, and projects. Woot! That area is relatively amenable to junior designers, and if you have your pick of several career options, I'd recommend an early jr. Designer run with a bigger company (more instruction, clearer paths to grow) and more mature jr. designer take on the smaller company/solo designer for a start-up route (forces you strengthen your development skills, which you'll be in a better position to do if you really have your fundamentals down. Not perfect, but down).
However... if this start-up is your only option, or were you in Little Rock, AR where there are few opportunities open to jr. designers, I'd recommend you take this position on the condition that a thorough investigation of the company and its people raises no red flags. If the choice you face is working at a start-up known to destroy designers versus working at a low-paying job and having freelance stuff on the side to up your skills, take the second option (your skills are more likely to improve, and you're less likely to face serious health issues).
Last note - on mentors: Like others have said, find several mentors, not just one. Recognize the relationships you have with them are largely dependent on your interest in growth and ability to organize your communicable goals, though they're even more dependent on how much time each mentor has to spare. If you can, ask people who have 1 or 2 years more experience than you for mentorship since they'll be more intimately familiar with the problems you're facing.