The ones I am using now say 0-9 months. I plan on using them until he can put a blanket on himself. He kicks blankets off so sleep sacks are great!
You're using a blanket in his crib? That can be a suffocation hazard so be careful. I'm glad the sleep sack is working for you as an alternative.
OP, we used a sleep sack with my oldest until he was in his big boy bed, so you can use them a long time. Covering themselves with a blanket won't happen for a long time!