Okay, so we know know the ages of the main cast, right? Sebastian's 40, Joseph is 33, and Juli's 27. But what we don't know is their DOB (date of birth). Using the magic of MATH (kinda sorry, but I'm very sarcastic), I have used a calculator to figure out their ages.
Now, I'm using the year "The Evil Within" came out, so 2014. Using this, we can figure out the years people where born. Sebastian was born in 1974, as last year, we are guessing he would've gone over the hump (hill? Anyways, he would've turned 40). Joseph would've been born in 1981. Juli, according to her age, would've been born only six years after Joseph, 1987.
Now, we can look at the age of Ruvik and Leslie, just for the hell of it.
I'm going to use the latter of the two, just because we don't really know when Ruvik was born (I'm going to call him by his birth name from now on, 'kay?), and we can guess Leslie's. As our poor Withers is only 25 at the events of the game, we can just use Juli's birth year to put him at 1989. Ruben, however, is a bit harder, because we can't say what year he was born in. So, I'm putting that he disappeared in 2002, because... reasons. Using that, we can say he was born in 1965.
Now, this is just speculation, not actual proof. So before you leap on me, screaming that this is wrong, at least understand that. So there, that's what I have. I may be popping up on other wikis, saying the same thing, but yeah. Have fun!