Why all dragon slayers age's are unknown? Please give me your comment please :)

I guess it is because they were raised by dragons who probably did not care of their age and the dragon slayers too forgot about them after being raised by those dragons. :)
I hope you like my answer !!! :)

I believe it is possible that there is more to this than we think. It's very possible that they are much older yet they don't know it. Remember at the beginning of the Nirvana arc how Natsu recognized Wendy? He never met her before, let alone seen her. It's also interesting how Zeref knew Natsu, but it could be something to do with a prophecy or something. I think there is a lot of mystery surrounding this question and that's exactly how Hiro Mashima wants it.

god knows

Well I guess u could say he's just a little younger than Lucy but take into account acnologia he was a dragon slayer but has lived for 100s of years so maybe they don't worry about age because they could live as long as the ancient dragon slayer art