With many teenagers and young learners the needs of the students are fairly standard. Often teenagers are in class in order to prepare for an examination or perhaps they are beginners starting out with the language and expecting to go through the school system and during their teenage years take an examination and "finish" English.
Adults, however, are generally more varied here. There are many, many different reasons why adults take English classes. Perhaps the most common is for business but there are certainly many other reasons:
Thus it is especially important to run a needs analysis with your class to find out exactly why they are there and what they will do with their English. There's no point in teaching extensive grammar to a class who want to be able to order food on their trip to New York, and there's no point in teaching someone how to order food when they will be using their English to telephone factory owners in Asia!
Plus a good needs analysis will also tell you what your class is interested in. Arguably the majority of teenagers are interested in music & celebrity so you can often almost guarantee a lesson based around music & celebrity will hold their interest (whether they love or hate the celebrity in question). However (again speaking very generally) adults tend to have more diverse interests and so there's no guarantee that the subject of your lesson will interest the majority of the class. Finding out what they enjoy will make future lessons much stronger.
Adults tend, on the whole, to want more value for money in the lesson. Unlike teenagers who don't pay for the lessons themselves, adults are probably paying their own tuition fees so they want to see they're getting what they've paid for. This means that winging a lesson isn't going to go down well. You need to plan well and make sure that each lesson is fully prepared. In other words, you need to be professional!
Research suggests that most adults have much clearer ideas about what they want to learn and are much more goal oriented than teenagers. Thus you need to make sure you give them what they need. Of course you can present material and have practice with all the resources at your disposal (video, internet, interactive whiteboard or whatever is at hand) but you must make sure they all focus to what the student needs.
One common issue with adults can be a certain reluctance to speak. In their adult life they may well hold positions of authority either at work or in their household, however in the classroom they are suddenly reduced to learners once more and this can cause some to be reluctant to speak for fear of making a mistake.
The answer is to make the classroom as safe an environment as possible and to never, ever, make a joke at the expense of someone's English. It can cause resentment towards the teacher!
It's often the case that adults are also working and have commitments outside the class. Whilst you need to make it clear that punctuality is needed, a little more leeway often needs to be shown to adults who turn up late.
Having said all this, the rewards of teaching adults are immense and it is not hard to build a great relationship with your students which can extend outside the classroom.