Firstly Taiji is enjoyable and can be practised at any level of fitness, so whether you have been regularly exercising for a while or have just started, Taiji can be practised safely. Taiji can be practised standing or seated. As with any form of exercise you will see the health benefits and improvements build over time. The more you put in, the more you will get out. A little and often is the best way to start. Taiji is does have to be practised regularly; attending a class for an hour once a week, although enjoyable, will mean slower progress than if you can practise between classes.
With regular practise the health improvements include
Taiji also can reduced stress and improves overall wellbeing and calmness.
There is a growing body of scientific research, mostly carried out in Australia and the USA, supporting these health benefits. See "The effect of Tai Chi on health outcomes in patients with chronic conditions: a systematic review" by Wang C, Collet JP, Lau J. Arch Intern Med. 2004 Mar 8;164(5):493-501 for a recent review.