My 20 month old has been wearing the weighted sleep sack since he was about 6 months. He’s now on XL, his 3rd size. His older sister also wore them for about a year. He’s a tummy sleeper so he wears it backwards. These sleep sacks have been a vital part of our sleep routine! I may transition him to the one with legs when he outgrows it, because I can’t imagine bedtime without dreamland baby.