Firstly, you can start dance classes no matter what age you are. Most schools will do classes for students from the age of 2 and many schools may offer adult classes, which are a fantastic way to keep fit.
You need to consider what style of dance you are wanting to take up and if you are looking to do several classes ensure the school can provide these classes. If you are unsure on what dance style you would like to take classes in, taster lessons are usually available allowing you to ay per class before committing to paying for classes per term.
Even if you are confident with what which dance style you want to take up i would highly recommend that you take part in a few taster sessions. This will let you get to know the dance school and their teaching methods. You may look round the studio and believe it to be the right place but the style of teaching might not be right for you.
Make sure the size of the classes are not too big, and if they are large classes they have more than one teacher. This is particularly important for young children as they will require more attention and one teacher will not be able to give enough attention to each child.
The biggest piece of advice i can give you is to shop around, every dance school is different, classes vary, the exam board used and what the school can offer you; from dance competition to end of year shows. Each person may be looking for something different so make sure you know what you want before fully enrolling in a class.