When training in the martial arts, you need to take your age into account. It is a major factor. If you are 40, it is unwise to undertake a system that relies upon strength, speed and fitness.
The external arts (karate, ju jitsu, judo, kickboxing, Thai boxing...) favour the young person. Strength, speed and aggression will work to the young person's advantage. You may find it intimidating (and demoralising) to be fighting a younger, fitter adversary. No matter how hard you train, your age will remain a negative factor. The risk of injury cannot be ignored.
Tai chi favours the older student. The subtle skills of the art require a mature, disciplined, patient mind. You focus upon physics, the application of pressure, sensitivity, rhythm, timing, balance and intention. Instead of wearing yourself out, you feel energised, relaxed and confident. You have a low risk of injury in tai chi, although bumps and bruises will occur in a self defence class.
The word 'fighting' has the connotation of reciprocity: two people trading blows. Taking turns. It is legally perceived as being mutually agreed upon combat. Both parties are involved in the conflict. Fighting usually involves emotion, stubbornness, pride and the desire to get your point across/have your way.
(iv) Self defence
The internal arts are about self defence, not fighting. Self defence is not the same as fighting. You have only one aim in self defence: escape without injury .It is not about winning awards and trophies or gaining a belt. It is not about looking cool or impressing anyone. In self defence you do only what you have to do and you leave immediately.