I started using these sleep sacks when my twins were about 9 months old and they were completely transitioned out of swaddles. I wanted something that would be like a blanket and keep them warm and also be safe for sleeping. I found these and bought 2 different colors. They are large and the twins were able to use the same sack for a year.
The smaller twin wore a 6-12 month size until 2 years old and the larger twin wore a 12-18 month size until 2 years old. Around 2 years old I bought the 18-24 month size and they still have room to grow.
These kept the twins from climbing out of their regular cribs, but not pack-n-plays. And keeps a great sleep association when it comes to bedtime. Love these.