Initially, you would need to obtain information about who was employed for the position instead of you and what his/her age and qualifications were as compared to yours. Most attorneys would not be willing to proceed with a lawsuit on your "feeling" of discrimination without some basis for you being able to say why you believe you were discriminated against or some evidence of discrimination.
Depending on the types of positions you were seeking and the company it could be difficult to find out who was hired instead of you. If you are not readily able to ascertain who was hired instead of you, you could contact the person with whom you were applying and see whether they would be willing to tell you the qualifications of the employee. You may also ask them state the official reasons according to them as to why you were not selected so that you have their official story as you proceed in your investigation efforts and deciding whether in fact you were discriminated against.
Additionally, you have said "long job search" and you should be aware that there are short statute of limitations for challenging the decision to not hire you, which begin on the date that you learned you were not selected for the position. You should file a complaint with the Washington State Human Rights Commission: http://www.hum.wa.gov/ComplaintProcess/Index.html as soon as you can to stop the clock from running down on your time to file.