I transitioned in my early 30s, and had virtually no dysphoria previous to that. But there were a lot of little things. They added up. I now realize I was female all my life, but I had been very good at suppressing that self-knowledge, probably until such time as I was actually ready to deal with my true self. Our brains can be amazingly adept at keeping scary or disturbing things off our radar... and once we become aware of them, it's like the subconscious decides that now's as good a time as any to start addressing it.
What I found immensely clarifying, though, was not pondering my identity, but asking myself how I felt about being gendered female (by myself and others) and comparing that to how I felt about being gendered otherwise. The former felt right, good, even euphoric; it felt true. The latter felt empty at best, but increasingly distressing and full of sadness, dismay, despair, anger, disgust, fear, and loathing. Dysphoric. Especially once I realized it would be possible to go all the way (which for me meant facial surgery).
I spent six months in therapy (while working on my voice and getting electrolysis) just to be sure of myself. Then I went full tilt, starting with HRT, and got everything done in less than two years. And now, almost twenty years later, I'm incredibly happy with my choices.
It's unusual, but not unheard of, to encounter these feelings later in life.