If you question your gender at all, you are likely some percentage trans. I didn't know I was partially transmale until 2013, and I was born in 1952!
My ex-husband recently emailed me that he also plans to transition to living as a woman, although he considers himself "other," or non-binary, gender-wise.
It seemed to come completely out of nowhere for my ex, but for me, it was when I asked myself what gender I considered myself to be, and was shocked when I heard myself reply "male."
Then realized I'd always known that. But I'm only 60% trans and have a very vocal female side also, weaker than the male personality.