I don't know the exact age when it first cropped up, but I'd say probably about 9 years old. For me it has never been a strong negative feeling, but a persistent almost wistfulness about being stuck in the male body I'm in. It always seemed like everything would be better without the parts. From my present perspective, looking back, I think that it actually was a more negative feeling than I recognized, but since it had always been there, discomfort felt like "normal." It makes me a bit sad to think about that, but it's been a good life so far nonetheless. For various reasons (i.e., wife and 3 kids), I don't have even the slightest wish that I could change how anything played out.
It's been all the more confusing because I've never been felt even the slightest sexual attraction to a male. If one is not critically thinking about the range of gender/sexuality combinations, that makes it hard to understand the feelings I was having.
Looking back, though, I have had lots of anxieties about mannerisms that I thought might be too feminine to be "ok." I.e., I have always crossed my legs and held my hands in ways that I feel are somewhat feminine. I was never quite what sure what else to do with my hands. I don't think there's anything telling about the leg-crossing or hand positions, but I think my hypersensitivity about trying to maintain masculine appearances probably meant something.
Anyway, here I am about 28 years later finally realizing that I can be me, even if it's not going to be easy.