what age?

To join any level of competitions, age is no bar. At any age we can start. It is our willpower, zeal etc which makes us success in any competition. Continuous efforts towards goal leads us success in life. Though it is also true that anything is started from childhood, there is more chance of success and very fast. If we start Swimming from childhood, say 4-5 years of age, there is more chance of winning the high level competitions, may be national or international depending upon our efforts, luck, support from others etc. I started swimming when I was at class first at the age of 5 years at pond. Though I am not competitor but can swim very well even now.

I think if we still fast swimming, we can do it however our age is old

I was a swimmer in highschool until college and based on my experience the best swimmers in our country started competing when they were just 6 years old, some of them learned the basic strokes by age 3. I could've been a part of the national team if I started earlier than 15 years old, because when I was still competing I often come in 3rd, the 1st & 2nd places are members of the National Team and I'm the 3rd, isn't that amazing! Oh well, it just feels great to reminisce but I hope I answered your question. One more thing, it also depends on the determination of the child to learn and to win the competition, there are many swimmers that started young but they only did because their parents pressured them to do so.

I started competitive swimming when I was 4 years old. It wasn't very technical or grueling at that point but I was taught the basics. As I aged and became more comfortable in the water my coaches stressed form and technicality more and more. I feel that the earlier you learn the better, it takes time to perfect being a competetive swimmer. However, some people are just naturals and pick up on swimming easily and wouldn't need as much time. It's never too late to learn but if you have the option I would say to start as young as possible.