I have reached the age of 'X', am I too old to start Taekwondo?

You are never too old to start Taekwondo, as long as you are physically able to train. Of course, the older you get, the more difficult it will be to win in competition for instance, but Taekwondo is much broader than sparring, something which people often tend to forget. The most important aspect of a training is that you feel good about the training and that you make progress on both physical and mental level.