Limbo was not just for unbaptized babies. It was for anyone who died with neither mortal sin nor baptism. It was supposed to be a very nice place, just not heaven. IIRC, you can only acquire mortal sin after the age of seven, but there are a lot of them, so in all likelihood, a 20 year old who dies unbaptized is in for a rough time. I haven't read the decision, but if the Pope declared ex cathedrea, then Limbo never existed in the first place.