Another factor is the number of tools and resources that are available now. When I first started designing crude games when I was in my teens, all I had was paper and pens - there was no Photoshop, Illustrator or InDesign, much less prototyping companies that can quickly and relatively cheaply produce working versions of a game on decent materials. Also, the existence of board game and board game design groups - particularly online - provide encouragement, support, feedback and advice that keep me motivated. I think we often forget how recent these communities are.
For me, the maturity factor covers an ability to plan, to set objectives, to remain focused and to communicate effectively (with collaborators, business partners, play testers etc), and having the experience to judge fairly well what works, what doesn't, when to stick to your guns and when to be flexible. It's really only in the last five years that I'd say I've been the kind of person who can do that more often than not.
Perhaps the most unexpected one, however, is my daughter. When I started drawing the first set of tiles, she saw me and wanted to join in. Of course I brought her on board as a design consultant right away, and we had a great time drawing roots, flowers and fruits. When I later introduced the bee, wasp and vole tokens she insisted on drawing them, and I'm disappointed that they can't really be included in a final version of the game - I love them.
She became genuinely excited about the game, and as a young child she was not exactly patient - so I had to develop it pretty quickly!
So of all the factors that contributed to Fruit's graduation from concept to tangible existence, perhaps the most important was my daughter. Never underestimate the manipulative power of a five-year-old girl!