My son was 22 months when he first rode this, which is obviously different than 5 months, but he was fine and a little baby would be okay in a sling or whatever. There's a lap bar that comes down, but in a Bjorn you could stick the baby's legs over that. Trying to think of the sling, hmm, with my son at 3/4 months I think we just pulled the bar as close as it would go when he was in the sling.
I don't think you could baby swap on it as there is no height restriction and they usually don't allow swapping in that case.