Developmental Milestone Source CDC Category Movement/Physical Development Expected age 18 No additional description of this milestone is available. Represented Developmental Observations IDOB#5019003 Runs stiff-legged IDOB#5261000 Walks up stairs without support, putting both feet on each step IDOB#5260004 Walks up stairs with support, alternating feet IDOB#5259009 Walks up stairs with support, putting both feet on each step