Wing growing is essentially biokinesis. It is using your thoughts to effect your body, which is a staple part of ALL wing methods out there.
I also incorporate magic and the astral plane into my belief systems, as I believe they play an important role in wing growing.
Can all people grow wings? The short answer is yes.
Long answer is, only people who have the needed determination, patience, and self discipline can.
People who are spiritually connected to wings, have astral wings, are angelkin/birdkin/had past lives as winged creature etc. have a much easier time growing wings, compared to a average person.
I disagree with the gene theory thing specifically because I know that genes don't tend to work the way this theory implies they work. This theory implies there are many different "races" or "species" of shifters which all just happened to evolve, and still were able to interbreed with humans thus creating modern day shifters and avians.
Evolution doesn't work like that. Evolution doesn't create wings out of nothing, or add an entire new organisms genome into an existing species' DNA (such as dragon and wolf shifters) Evolution would have changed the arms into wings, because evolution always takes the shortest route. It has never added new limbs on, at least not in the kingdom Animalia.
I firmly believe that anything is possible given proper research and experimentation and unfortunately I also believe that prescribing to one specific idea and never straying is what inhibits the progression of both the human and nonhuman community.
You have to go about things with a very round sort of mindset. By that I mean, use multiple philosophies on shifting even if they are counter intuitive or contradicting at base. You might find yourself getting farther than you ever have before. Use the metaphysical and the "factual sciences."
Otherkin, shifting, spirituality... these things that we connect with and are who we are, are so individual. The inner journey is yours alone, and I would encourage you to challenge it, question it, explore and pursue it. I think exploring what is already inside of you will be much better than looking for proof of shifting.
Wings take many years and cell-manipulation to grow. They take many weeks to months of mental conditioning and brain-work to get them to want to grow (which also the activation of the gene, as it takes a while to fully activate.)
What really happens is simple- you tell your brain to grow them, which in turn tells your stem cells (yes, teens and adults do have stem cells) to start to grow into the wings.
If you are an aviankin or wingedkin, you know what you are supposed to have. Your soul, your body, it knows what to grow.
Wings take forever to grow, wings take upwards of two years, and if you give up you will stall and stop them completely.
Mental changes when growing wings are often slight, but i have noticed the following:
-Staring up at the sky
-An inner voice telling you to fly whenever you’re scared or nervous
-A call to the sky, as if you belong there
-Dreams of you with your wings.
-Developing permanent astral wings and/or the strengthening of your astral wings
-Your wings possess your arms so they may move freely and be seen
-Your wings become a part of your life and you feel as if you need them to be yourself.
Supernormal: Science, Yoga, and the Evidence for Extraordinary Psychic Abilities Novel
Psychokinesis: Another Example of How Consciousness and Reality Are Intertwined