A lot of this has to do with energy and enthusiasm and not age - though I do take the point about age and in my twenties in Thailand I found it easy to get work.
What I saw over the years in Thailand though were a number of older teachers who had, over the years, gained a lot of experience but somehow lost their love of teaching along the way and had failed to realise it or, if they had, they were often in denial about it. But this much was clear from the way they conducted their lessons - well structured, linguistically meaningful sessions, as their experience would suggest, but conducted on auto-pilot - as well as their general conversation relating to their profession which lacked passion or was even downright negative.
Students and collegues pick up on this and often, no-doubt unfairly so as regards well qualified, experienced older teachers, this leads to the hiring manager not wanting to make the same “mistake” twice. This is like any hiring situation - it’s risk management and the perception of risk is often rooted in past experiences, whether the fundamentals upon which this belief system is underpinned by are real or imagined.
Let’s also remember that it’s all very well comparing a 20-year old layabout to a 50-year old with a degree in education but I also encountered a lot of 50-year olds without a degree in education - or a valid TEFL qualification for that matter.
Indeed, I remember one such 50-year old criticising me to my colleagues behind my back because I had chosen to go and get my CELTA. In fact, I would go so far as to say that the older teachers saw less value in professional development and had a track record that struggled to point to any - after all, they are native speakers - what possible benefit could they have gotten from a TEFL course?
Also, a lot of older teachers wouldn’t teach if they could get away with it. This, I find is less pronounced in younger teachers, who are still enjoying finding their way in life and collecting experiences - not a bad thing as long as they are doing so in a way that facilitates their learners’ development.
I also encountered seasoned teachers but their enthusiasm had gone or they were wanting to slow down. Trouble is, your audience doesn’t get old with you - it rejuvenates on an annual basis and is always going to be 10 years old or whatever age group you teach. So whilst that combination of qualified, incredibly experienced and enthusiastic does exist - it is less common than you might think.