Adults = brain full of data, very low brain elasticity/plasticity compared to kids => You have to psych them out, get into their brains through trial and error (if you have a better way please propose) evaluate their strengths weaknesses and learning abilities and eventually create an approach that you believe is the best.
Kids = zero data, super duper brain elasticity, the teacher is god because they are taught so .
Definitely, in most occasions, its easier to teach young kids the basics no doubt about that (but then again, a kid with no talent at all maybe harder to teach than a talented adult, rare occasion but it happens ).
When it comes to polishing the overall attributes of the player to reach a semipro or pro level then its definitely a matter of good communication in both occasions.
The hardest thing for me is to persuade a player about what is best for him/her, especially when he/she believes otherwise. Kids are easier and learn faster, their brains are blank.
An adult with a "bad" habbit needs to erase it and start from scratch , but first you need to convince him to erase it and you have to be sure (which means lots of time "Wasted" on trial and error tests) about what your training/communication approach will be for the player to evolve. I dont find it surprising that good coaches damaged adult players. When I returned to TT after 15 years break I got much worse through training because I was listening to every advice on technique and rubber from everyone, didnt have the necessary experience to filter which advise is good or not.
So a great factor is the player's self awareness on his playing strengths and weaknesses. A talented kid with super duper FH has almost 0 self awareness, most of them cant actually explain/grasp WHY their FH is so good. And you also have guys with crappy body mechanics that think copying ma long is easy and even more funny they actually convinced themselves to believe that they need this kind of FH, but the truth is they want that kind of FH because its flashy.
Coaching adults is way harder in my experience, but also way more rewarding in terms of self affirmation. I feel like I can control a giant puppet when things go as expected/planned but the dissappointment of a wrong approach/advice is also very frustrating because all the hard work has gone into the trash can and at least for me is very tiresome because you must always reassess and re evaluate your thought process to adapt to the player's needs