I entered my birthday and it said that I'm 17 though I'm 18, how come?

[mesquitebean]

Are you working on a 2018 return?   
Did you turn 18 during 2018, or was it in 2019 after January 1?  
And what screen are you referring to as "it?"

[boigotdough49]

Your personal info.

[boigotdough49]

I turned 18 in 2019

[boigotdough49]

and yes I'm working on a 2018 return

[Bsch4477]

Then you were 17 in 2018.

[mesquitebean · Accepted solution]

When preparing a Federal return, the IRS bases the calculations on your age on January 1 of each year.  So for a 2018 return, TurboTax would reflect what the age was on 1/01/2019.