I think it is possible. Because the internal style can become the complementary training to external style. Usually, the practice of external style is a bit stiff, people focus on physical training too much. Internal style will give them an internal training which can perfect his external practice.
In our school, there are a lot of students who have a long time experience of practicing external styles, and, so of them have very good foundation through their external styles' training.
And, there are some students in my school still take some other training such as Shaolin, Karate, Judo at the same time. I told them, you can take any training when you will you can make a progress. And, I believe, different style will give people different benefit. There is not any conflict between different styles because all of the styles are for the benefit of our human being.
Everyone can study internal martial arts no matter how old he or she is. Because there are two ways of internal style: first is internal power. Second is self-defense. People can develop internal power even though his physical situation does not permit him to develop the self-defense ability. So, the age is not a big problem when people take the training correctly.
In internal style history, there have been so many good masters who began to study the style when they were not very young(some of them older than 40). In the end, some even became a Grandmaster. Because this style is different from external style which focus on physical training mostly.
And, for Qi Gong training, because it is for health and good willing, there is no age limitation.
Please remember: people will become old only when they think of that they are old by themselves. So, do not make your young-age become old so early. And, you will feel that you are becoming younger are you practice it. The only reality is that you will be one day older tomorrow than who you are now, so we should practice more TODAY before we are older TOMORROW.
Qi Gong can be used as self-defense. But, that should be martial art Qi Gong. Zhineng Qi Gong and Anti-Caner Qi Gong are for health, not for self-defense.
However, most styles of Qi Gong can help people get internal power which will help you for self-defense when people use it properly. So please do not seek direct self-defense benefit from doing Qi Gong.
In order to answer this question, we should know what is internal power. Usually, people use some descriptions to answer it. But, that answer is only the effect of internal power.
In my opinion, the internal power is the combination of breathing method(make the external Qi work for internal Qi), meditation skill(for developing the mental force), correct posture(give the channel for Qi to move smoothly and make your physical body to work) and self-defense technique(internal power can not work for self-defense without skilled technique).
Internal power is not magic, people can feel it after they get it, and, people can use it naturally when they practice it.
So, how to get it?
I think I have mentioned the answer above: breathing, meditation, posture and technique. People should always remind himself during practice according to the four important points.
To be straightforward, I do not know the answer since the term of mastery is not clear. The concept of mastery can be pretty complicated.
But if we make it simple, or let's say that people want to know how many years to get the important practice of each style, I can say that it takes at least 10 years time. For myself, I do not think I can finish studying these arts before I die. The most important part of practice is not the destination but it is the journey.
In my opinion, martial arts and Qi Gong is only a kind of sport or exercise which has a special effect. Although they are practiced by monk, or priest, they are not religion. Originally, people used etiquette in martial art training. But, that's only for respect , not for religion.
And, in Chinese internal style, we study philosophy in order to make our martial arts' practice better. Because internal style has very closed relation to Chinese philosophy, such as Yin Yang and the nature. But, philosophy is not a religion. Chinese philosophy focuses on the way of life. I think most people understand it.
And, I do not like putting martial arts and religion together.
It depends on what you want to get, or what you want to do. Because practice different style will give you different benefits.
Xing Yi is good for developing internal power and visible force. It has a very outstanding self-defense feature. And, there are less forms in Xing Yi compare with others. Because Xing Yi focus on developing internal power. People will get self-defense ability in a short time after they practice Xing Yi. And, to any style, no matter internal or external, Xing Yi can help them to enhance power.
As for Chen Tai Chi, it is very helpful for people to understand and get the special Soft-Force, Silk-Reeling, Absorbing the Force from the opponent, getting the flexibility of the body and Chinese fighting strategy. This is a very deep style, not only for health, for self-defense, but also for life, this is the way of Tai Chi. So, people should select this style when they want to contact with Chinese internal martial arts.
As for Ba Gua, it is a very useful fighting style. But, it needs some martial arts' background for the person who want to do it. And, Ba Gua has a lot of body work, walking, turning. So, this style is for the person who want to make their present training better, no matter which style they are doing.
As for Qi Gong, Zhineng style is developed for health, for prevention and healing of illnesses. So, Zhineng Qi Gong is suitable to everyone.
Besides, my advice is, when you want to select one style, please try to do some research on the Internet, or read some books, or visit some school. Try to know the basic knowledge of this style, then, try it. And, my experience is: follow your heart, your feeling will give you a good answer.
This is a good question, and many people asked me this question because people want to know the lineage of the teacher. In Chinese martial art culture, before decide to learn from someone, you have to ask that person's lineage, and at least trace back to 2 generations.
I have many teachers, but, my first teacher is my grandfather(Yang Qinglin, a very famous Xing Yi-Ba Gua Grandmaster in China, he was the students of Zhang Zhaodong who was the founder of Xing Yi-Ba Gua palm). And, most of my basic training of Xing Yi and Ba Gua and Tai Chi was for him when I was 8 years old. I studied Chen Tai Chi from my uncle, Yang Fengwu(a senior student of Chen Zhaokui who was the son of Chen Fa'ke), my uncle was good at Fa Jin and Tai Chi application which is considered as the secret of Tai Chi. In order to get the advantages of different internal styles, I visited many masters in China and exchange the martial arts' knowledge with them. Because there is a famous martial art proverb"When your Kungfu is good, finding a good teacher is not as good as visiting your martial art friends". And, I still keep the contact with many of them in order to share our knowledge.
In order to help me to study more, my grandfather and my father introduced me to study from other masters such as Hu Jinling(Xing Yi master, student of Han Ziheng, grandstudent of Li Cunyi), Fu Shoubo(student of Cheng Youxin and Cheng Youlong), Zhang Sonlin(grandmaster of Xing Yi, good at explore force). Ma Hong(grandmaster of Chen Tai Chi in China).
For my Qigong Studying, I studied different styles. I studied Zhineng Qigong from the founder, Dr. Pang, so I got the certificate which can show my training and level(The number is 000340). I studied Daoist meditation from Yang Meijun, the grandmaster of Wild-Goose Qi gong and Daoist meditation, from Cao Zhenayng, the Taoist priest in White-Cloud temple of Beijing. I studied Potential-Ability Development Qigong from the founder, grandmaster Li Shuhe in Tianjin. And, I studied Qi Gong from my grandfather, my uncle and some other teachers too.
As my TCM(Traditional Chinese Medicine) training,in the beginning, I studied it from my uncle. Then, I went to school to take the formal training in Tianjin TCM University.
Right now, my teachers are these masters who are living in my heart, even some students can be my teachers since I have learned how to teach in the West from them. So I can say that there is no last teacher, there is always something for me to learn and study.