Often we face this problem, there are may example on google, but still i feel like sharing this with you, this is how you can play with any date format in any culture.
DateTime dob = DateTime.Now.AddYears(-18);
IFormatProvider culture = new CultureInfo("en-US", true); // this is how you can set any culture you want !
dob = DateTime.ParseExact(txtDOB.Text, "dd/MM/yyyy", culture);
profile.DOB = dob;