Child developmental research clearly shows that children's language acquisition capabilities are greatest before puberty after which brain organization changes making natural acquisition more difficult. Children growing up in an environment where two or more languages are spoken are not only able to acquire each language easily but also able to completely separate them.
This is a generalization of course. Children's cognitive abilities differ and develop at different rates. Some adults retain the ability to acquire any new language when immersed in it at very fast rates.
However, starting in kindergarten or even earlier is a great idea. There is also research that shows the multi-lingual skills acquired in childhood has a very beneficial effect on brain development that is used in other areas.