Is there a time frame that a company has to come after you about a debt? I am being asked to pay immediately a debt from a college in 1999. Do I have to pay this, and if not, how do I go about it.?

is there a time frame that a company has to come after you about a debt?? I am being asked to pay immediately a debt from a college in 1999. do i have to pay this, and if not, how do i go about it.??
And what would cause an acknowledge of debt?