We don't know the average life expectancy of any of the races do we? cause say its like mid 1700's kind of thing where most people are considered very lucky to make it to 70 if they don't access to expensive medicines and magic.
This is a common misconception regarding the medieval ages. The average life expectancy was lower, not because most people died in their 40s, but because of high child/infant mortality rate. Being active (as opposed to sitting on couches all day) and not being so regularly in contact with carcinogens and greasy food, people in the medieval ages often lived past 70.
With humans (and possibly elves and dwarves), I'd imagine they kept age expectancy aligned with our medieval ages, just so that we don't have to really think about it, so we don't have do any mathematical conversions.
Does this mean I can't be a few years over 40?
Must I be under 40? Can't I be 41 or 42 or even 43?
I'm guessing that 40 is not a hard limit and that the actual dialogue/plot elements of your origin will be ambiguous enough regarding age to allow you to roleplay older.