I’ve worked with and trained recent graduates who were able to manage classes without raising their voices and middle-aged former project managers who went blank in front of a group of expectant students. Most TEFL training courses will not accept trainees younger than 18 but there is no upper age limit. However, the demands of the job probably rule out all but the sprightliest of octogenarians. Basically, anyone who is old enough to work is probably capable of being a TEFL teacher. Sally, who I mentioned earlier, was in her early 60s and used her wisdom and experience to great effect.
On the other hand, it is also true that some students may prefer teachers of a certain age. Teenagers may respond well to younger teachers at first but can change their attitude if you try to be their friend. Mature students may feel dissatisfied with teachers who are substantially younger than they are. Older businessmen, for some bizarre reason, often like to learn from young women who think a hedge fund is the money you save up to buy a new lawnmower. I think it’s fair to say that first impressions do count and younger and older teachers might find themselves victims of prejudice in certain contexts. On thw whole though, competent professional teachers should be able to overcome any initial student scepticism after delivery a few solid lessons.
The nationality of a TEFL teacher is more problematic and revolves around the native versus non-native teacher debate. I have trained Brits, Americans and Australians and some of them were excellent teachers while others struggled to spell their own names correctly on the whiteboard. I’ve also trained Spanish, Italians, Brazilians, Germans, Dutch, Danes, Hungarians, Japanese and other nationalities. And you know what? Some of them were excellent while others struggled. To become a TEFL teacher, you need to have an excellent command or spoken and written English (C1 /Advanced level and above). Beyond that, your personal qualities and teaching skills are what really matter.
Unfortunately, the industry – influenced to a large extent by students – seems to prefer native speaker teachers. This means that excellent non-native English speakers are overlooked in favour of mediocre native speaker teachers of English. I know of a number of non-native teachers who are less than 100% honest when it comes to revealing their nationality to their employers and students. It’s amazing how many of them suddenly discover English, Welsh, Scottish or Irish parentage!