I've heard before startups saying "yeah, but once you're a bit older you can't commit to the 14 hour days, you have family and stuff". 14 hour days are bad and wrong anyway, no one should do them, and no one should expect them. Grumpy grownups have probably done them before, but have enough knowledge of the languages that the 14 hours can be squeezed into 9.
Anyway, startups should hire the right person for the right role. Is that the 21 year old kid who can sling code like a ninjarockstarboss and cut her teeth when she was 12? Maybe! Or is it the 40 year old dude who's seen the rise and fall of titans and has 10 years experience in the domain? That depends entirely on what your business needs when you hire someone, and what the business is going to need long term.
If you're expected to work 14 hour days as anything other than a one off then it's not sustainable. Don't get me wrong, some developers are very capable of being able to grind 14 hour days out over and over, but for the average person your quality level is going to dip off rapidly, you end up overly stressed and it ruins your health. I say this as someone who has gone his time in the pits doing 14 hour days, and whilst I throw the occasional one in when needed it's a rare outlier.
S = number of hours an employee needs to sleep to be useful (this varies person to person)
C = commute time to the work location
M = maintenance time (how much time a person needs to do minimal human maintenance, eating, relaxing, showering etc)
So the upper bound on N should be:
N <= 24 - (S + C + M)
Now this doesn't take into account burnout etc. So another factor B, should be put in the equation. It represents the time an employee needs to spend on non-work things (hobbies, family, friends, going to the pub, whatever) to refresh themselves for work and prevent burnout.
The average person seems to have these sorts of values:
S = 8
M = 2
C = 1
so assuming no B (extremely unlikely), this puts 13 as an upper bound on N for long term. However a normal 13 hour day is a LOT. I know for me B is around 2 or 3 and a lot of people think I'm nuts on how much time I spend doing work. I should note, in my case, B is actually higher, however aspects of my work also count towards B - I love my job, and so can spend more time at it in a sustained way because of the "fun" or "paid hobby" aspects of it.
For N <= 8, making allowances for lunch hours and breaks.
Short periods of time (measured in days, not weeks) can be spent working for N > 8 hours per day, but it becomes counterproductive (from the employer's business standpoint, not even considering employee health or satisfaction) beyond a few days in a row.
Anyway, startups should hire the right person for the right role. Is that the 21 year old kid who can sling code like a ninjarockstarboss and cut her teeth when she was 12? Maybe! Or is it the 40 year old dude who's seen the rise and fall of titans and has 10 years experience in the domain? That depends entirely on what your business needs when you hire someone, and what the business is going to need long term.