Medical Bills from 10+ years ago now being collected

[Tygger]

Hi! I live in New Jersey and have a problem. Last year a local hospital closed due to bankrueptcy. Well now myself and many others are getting collection notices for services done at this hospital as far back as 1990. My husband got one for an ER visit in 1998. Now they are saying we owe $957. We never got any bills for this just this collection notice. Other people I have talked to have said the same thing no bills just collection. After much calling around to the collection company they basically have said they are collecting on the difference between what was billed and what the insurance companies paid them. We also got one for my husbands deceased grandmother dating back to 1990 for $14.80. Is this legal and can they go back that far? What is the statute of limitations? Thanks.