Most HM start out as deltas and as the males (and females) mature and stretch their webbing they become HM. I'm currently trying to get one of my fry to go HM, he's getting closer. By letting them flare every day for about 10 minutes they can stretch the webbing between their rays.
After a certain age... if they haven't reached 180 degress.. my opinion is they aren't going to most likely.
I'd be interested to hear Martinismommy's thoughts and how she gets so many HM in her spawns. I have a feeling there's a lot more genetics than people think and I also think it relies heavily on the female. A female with heavier branching and more spread matched with an HM male should yeild a much higher percentage of HM fry.
*Be smart.. Do your research!!*