The youthful tone that most of the writers here choose to maintain has everything to do with gaining their clients.
I don't do anything to alter my normal patterns of expression and I've been very open about the fact that I don't use Facebook or Twitter or text, and that I don't even own a smartphone. None of that seems to have affected my ability to get new clients. Quite the contrary, actually. Plenty of my clients have specifically mentioned that they chose me based on the substantive content of my forum posts and on their comparison of the way I express myself in written English to the way other writers express themselves here.
It will be easier for a student to trust a writer who sounds young and up to date as opposed to a writer who sounds serious, businesslike, and older than the client.
That hasn't been my experience at all. I don't know where you get your information or why this conclusion makes sense to you, but most students probably prefer doing business with an experienced, serious-sounding and businesslike writer who's been in this business for decades and who's produced about 10,000 essays during that time than with someone who was still playing beer pong in a fraternity until recently and who's really not much more experienced writing essays than the typical client.
An independent writer should know how to sound young when required, and older, if need be. It is the only way that a writer can remain competitive in this business.
I've managed to remain competitive in this business without doing anything to pretend that I'm younger (or older) than I really am. My guess is that most clients just want to deal with writers who aren't misrepresenting anything about themselves. Even my website and my advertising material mention that I've been writing academic essays professionally for 20 years, which makes it quite obvious that I'm well over twice the age of most of my clients. My guess is that they're much more likely to consider that a good thing that inspires some confidence rather than a bad thing that alienates them.
Although the writer is aging in real life, his writer age must remain stagnant / non- determined for as long as he wants to remain working in this business.
That's probably true for actors, not for writers.