What is important in choosing a Ph.D. program?
The students in this lab are all from top universities

it is an irrelevant criterion for choosing a Ph.D. program for your

... not as interesting

Unless you don't mind being miserable throughout your P.hD, you shouldn't work in this lab if you don't have to, which seems to be the case.

... the professor is very young

I think it is a plus, instead of a negative.

the students are so hard-working that the work-life balance might be bad.

Just because they don't know how to balance their work-life with their life outside of work, it doesn't mean you won't be able to too.