What is the best age to start learning the language?
Ideally, when they learn to speak. After that, any time is good. Children have an incredible ability to learn new words between ages 2 and 4; these are years of rapid growth in their vocabulary. At this time, they learn without much discernment as to whether they are learning Spanish, English or Chinese. They just learn. Once they learn reading and writing, they help themselves with these tools. They may need to interact and practice more to internalize the second language, but they are as apt as when they were young children. They will be at one level in their native language and at another with the second language, but fluency and proficiency will come with regular use or practice.
