Milestones are skills that children usually develop at certain ages. For example: smiling at 4 months, reaching to grab a toy at 6 months, or walking at 15 months. Each child develops these skills at different times, but if they are far behind the expected timing, it’s good to check on their development with their doctor. If a child misses milestones, it may be a sign of developmental delay. More about milestones in child development.