Generally the period from 3/4 years of age to 12/13years is considered as pre teen or young age. After 3 or 4 years of age the physical changes in child do not come as quickly but that does not mean that the changes stop completely instead it continues with slow pace. The reason why am I writing this is to make you understand that the most favorable time to keep pictures up to date for the portfolio of your young ones for modeling purpose is after 3 or 4 years of age.
There are many activities that parents induce their children to improve their certain extra abilities. And most importantly the happiest and the most memorable moment for all the parents is when they get the praise for their child's performance. And fortunately there is other way round from where you can bag several round of attentions, rewards and applauds and that is by allowing your baby to pose for an advertisement or for a role in a commercial. Modeling is one such occupation you should let your child try if your child is cute, charming, expressive and talented in every way. So for that reason you need a portfolio.
Looking at the different advertisements it can be assume that there is no set starting age for children to start modeling but from the portfolio perspective parents should try to be very practical about portfolios. It's a bit difficult to keep pictures up to date because the physical changes in babies take place very quickly. For instance take case of a baby's teeth, hair, smile, that keeps changing with the time, so the recommended age to start a portfolio for modeling purpose is around 3 or 4 years of age, after that physical changes do not come as quickly. The kind of pictures that is required is simple and for that you do not need to hire any photographer. You can do it yourself. Shoot some simple headshots both indoor and outdoor that will do the trick. Generally ad agency selects babies based on the impressive headshot and smile. Therefore avoid using too many props, makeup, that will divert the attention of the casting directors because it is not in your knowledge what the casting directors are looking for. Avoid using sunglasses and hats or anything that may cover the face of your child.
Everything is fine if your choice matches with your child's dreams and desires. Therefore, before sending their pictures to child modeling agency, it is your duty as a responsible parent to have an open conversation with your child, even if your little one is too small to make their own choice, because what appears to be great hunting ground for the ambitious grown-up can be tedious and sometime even detestable activity for a child.