Babies start to understand the concept of stacking around 12-15 months. At that point, many kids' brains, eyes, and hands are at the right development stage to start with this type of play activity. When you're looking for blocks to start out, open-ended blocks tend to work well, rather than block sets for constructing specific objects (the latter are great for older kids, though!). Young toddlers learning to throw things can safely play with large foam blocks available in various colors, shapes, and sizes. Older toddlers tend to have fun with cardboard or wooden block sets. As your child gets older, you might move to more complex sets such as Waffle Blocks and Lego Duplo. Just remember that sets can sometimes limit kids when they're just getting started with blocks, frustrating children not yet ready to move into more complicated building activities.