newer metal IMO is like post 1995
Metal took a different turn around that point, maybe a few years earlier, but the sound/bands have never been the same.
Bodom formed somewhere around 93 and in flames was in '90. LOG and a lot of other bands like to call 'classic metal' formed later. Classic metal for me is something like metallica, but the term doesn't mean much. A lot of north americans qualify 'classic metal' or bands that play something close to 'classic metal' just by them being thrash-ish.
New metal or 'nu-metal' is something like what limp bizkit, korn, slipknot etc. (although slipknot shouldn't be there)