The age at which babies start standing varies considerably but it is usually between 10 – 18 months.  When your baby starts standing you should lower the crib mattress.  Her head should come just above the crib rail when she’s standing. At this height she won’t be able to pull herself over the top rail which could cause injury.  Also if your crib has bumpers at this stage they should be removed as she could use them to climb up and over.