The critical period during infancy lasts up until the age of 2 and a half years old. Bowlby originally said that if a primary attachment is not formed within this window, an attachment could never develop and thus the infant would never be able to form healthy relationships later in life. Counter examples to this made him change his idea. It became a 'sensitive period' that could last up until 5 years of age. He said attachments could form but take more effort/take longer the later they occur.