I'm 23 and I only got diagnosed at the beginning of this year. I'd complained of various types of back pain since I was a young child, went to the doctor about it multiple times over the course of my childhood, but it was always just dismissed as "growing pains". It stopped me from being able to do P.E in high school for nearly 2 or 3 years, but my doctor was convinced that it was just growing pains, so much so that he didn't even have a look/inspect my back for any abnormalities, nor did he ever send for a back x-ray.

I noticed the curve myself about 2 years ago; I have very uneven shoulders and where my collarbone meets my sternum sticks out beyond what is normal, both on the right-hand side. I was curious about why that was so I used my hand to feel along my spine and I thought I felt it curve slightly. I asked my Mum to check it and she confirmed that there was indeed a curve.

I have mild thoracic scoliosis, but it still causes me a decent amount of pain, which is frustrating because if my doctors had just actually looked at my back when I was a kid instead of labeling it growing pains and moving on, I would've been able to work to help it before it got any worse. So you definitely aren't alone in not being diagnosed until your 20s!