How long your baby is in each size will highly depend on their size. Some babies stay in sizes longer than the recommended age size, while others will grow out of the 3-6 months size by the time they're two months old. As much as sizes are labeled by age, they are based on weight when it comes to baby clothing. While there isn't much of a weight or length difference between sizes, you need to remember that 3lbs is a big difference when it comes to babies. A baby that is born weighing 7lbs will have increased by 30% when they reach 10lbs.