Putting
Textbox1.value
inside an SQL query string does not make any sense: the query gets executed by the database, which can't access your textboxes.
The logic I would suggest goes as follows (in pseudo-code!):
DateTime now=DateTime.Now;
DateTime minDOB=now.AddYears(-textboxMaxAge.Value);
DateTime maxDOB=now.AddYears(-textboxMinAge.Value);
string query="SELECT ... WHERE DOB between ? AND ?";
AddParamWithValue("?", minDOB);
AddParamWithValue("?", maxDOB);
execute(query);
:)