On average babies double their birth weight by 4 months; triple their birth weight by 12 months and will be around four times their birth weight by 2 years. (Some babies will reach these levels at a much younger age. Some at a much older age. The most rapid period of growth is between 2 weeks and 6 months of age. This rate of growth decline slightly at around the age of 6 month; declines further around 9 months and declines further still at around 12 months. (These ages can vary depending on each individual baby.)