You’re right except for implying that genes are necessarily more active in an adult.
Gene editing is often performed in the zygote or few-cell embryo stages because of the ease, reliability, and ability of those cells to withstand the procedures used for injection or uptake. But, this alters the germline of the organism, where offspring of successive generations will inherit the genetic alteration.
Gene editing in organisms comprised of trillions of cells and fortified extracellular space is far more difficult and the methods of injection, electroporation, and treatment with chemicals like ethylene glycol (antifreeze), aren’t practical.
Instead, viruses stripped of their ability to use host cells to replicate and stripped of their antigenic (immune-system stimulating) properties, or nanoparticles with an attached payload, are utilized for delivering whole-organism genome editing systems to many cells. He Jiankui probably used one of these options to edit the genome of a 14-week old embryo, but the article doesn’t go into any detail.
There are at least 3 categories of ethical quandary involved with human genome editing.
Is it okay to alter the human germline (unnaturally-induced heritable changes)?
Is it okay to attempt to improve the human genome? (What exactly is considered an improvement?) Increased intelligence, athleticism, disease resistance, attractiveness. Some of these are subjective and may cause unintended harm.
Is it okay to use genome editing to cure ill health? If so, what is to be considered “ill health” worthy of this type of treatment, and what isn’t?
We would most likely fall into the third category and I have no doubt that once the tech is reliable, there will be amendments in laws to allow medical treatment and professions dedicated to it. Also in our favor is the likelihood of this being an epigentic condition that could be treated with epigenetic applications of CRISPR and similar technologies, avoiding some of the considerations given to changing the underlying genetic sequence.