In Mexico the American teacher is preferred. In Indonesia a school may prefer the Australian teacher. In Europe British English may be preferred and within the European community teachers from the UK may be the only teachers hired. You may be teaching children, housewives, college students or corporate executives. It will depend upon your employer.
Most language schools enroll students of all ages and backgrounds.
Your income is relative to your position, qualifications and destination. You should consider the cost of living of the country you are heading to. In China, for instance, you may enjoy a high standard of living but probably won't be able to pay a large mortgage payment back home. Many teachers make extra money teaching additional classes or taking on individual students for tutoring. In Korea and Taiwan many teachers make lots of extra money tutoring, but many of their employers forbid this.