Is it possible to not be diagnosed with ADD as a child, but later in life? How will it effect my school/life?

Yes it's possible. There are a lot of people that are diagnosed with ADHD at different ages. That is not uncommon. I find that it is over-diagnosed in children though, and that greatly disturbs me. I was diagnosed with ADHD at 12 and I was on Cylert which from what I heard was recalled for a while. Not sure if it's back on the market or not. I was only on it for 6 months but due to bad reactions to it, I was taken off and never put back on something or treated for it again. Then in college, I was doing very well in most cles that involved critical thinking, but when it came to cles like math and music theory which required ytical thinking, I was getting Ds and Fs and they couldn't figure out why. My music theory professor told me the only way I could take that cl a 3rd and final time was to first go to Psychology services and get tested for learning disabilities. Results came back as ADHD, Anxiety disorders, Depression, PSTD, and a lot of other issues. Anyways, they put me on focal-in which I had a bad reaction to, switched me to ritalin and again bad reaction, so instead of medication, they had me working with a therapist as well as academic accommodations. That's what got me through.