A baby should not sleep with a security blanket until around 7 months of age to reduce the risk of SIDS.
You can begin introducing a security blanket around age 3 months however don't let the baby sleep with the security blanket until around 7 months to 12 months of age.
After 12 months the baby can sleep with most soft toys including the security blanket without risk of suffocation or death of SIDS.