Scott - John Cusack
Rob - Brad Garrett obviously
Dean - maybe Depp or Ashton Kutcher or the guy who played Vader in the star wars prequels.
Eric - that blond headed dude that always pops up in sitcoms and movies. He starred in the dumb and dumber sequel.................Eric Christian Olsen (thanks wikipedia). For being in so many things over the years, this guy is pretty forgettable haha
It's hard to pick actors based on age because STP has been around for so many years. But I think with the right makeup, these guys could work.