That is a harder question to answer than it seems.
At what age did I know for sure? 61.
At what age did I start to wonder? Some time in my 40s, I suppose. I talked myself out of it. Nope. Couldn't be. No way.
At what age did feel something that, in hindsight, I could identify as dysphoria? 17.
At what age did I have thoughts that, in hindsight, can be identified as trans? 7.