80s are 1990-1997, 100s are 1998 to 2007. They don't suffer with rust but most are of an age where if they haven't had some form of rust prevention at some point in their life they will have problems. Mechanically wise they will last forever. There are 1HD-FT and FTE trucks in Australia with 3 million Kms and no rebuilds. new injectors every 1.2 million km, and front axle rebuilds every 250k miles ish (on the 80/105 series)What age is old? Anything before 2005 has been welded here, maybe it's the salt on the roads
The 12 valve (1HD-T) isn't quite as sought after as it needs big end bearing done at about 200-250k miles. Sorry for mixed units