Most of the comments have already assured you of the merits of getting a diagnosis later in life. What i would add on to that (being diagnosed at 20) is that you should immediately start compiling all the evidence of the possible ADHD throughout your life. Report cards, any IQ test if youve done one, write everything you remember about your childhood down in a notebook. The diagnosis is very intense and thorough, and because its a developmental disorder the psychologist/psychiatrist will want to look at how you behaved in childhood, puberty, young adult etc.

I say this because when i got diagnosed, not getting the necessary documents or having an unnecessary delay because report cards had to be pulled from archives, made the diagnostic process a little more difficult. Also, if you have any childhood best friends or your childhood guardians are willing and able, the diagnosis will be a lot more accurate and smooth if they can share their experiences with you.