Level 90s mentoring down to the lower tiers are designed to be essentially running in godmode. The original point of the whole thing was essentially about powerleveling up a friend who's come to the game and catch them up to your level. It's also evolved into a means by which players can solo through lower tier heroic content to get AA. In any case, if you're looking for that "original" experience, then don't bring a mentor into your group.
That said, I'd also point out that even if everyone is in their low 30s running through Nektropos Castle (for example), there's really still not much challenge at all to the content. Players are much more powerful than they were 5 years ago at the same level, and that old content was never adjusted to account for AAs and the vastly superior gear that everyone has easy access to (with stats and bonuses that a level 50 raider would have drooled over back in the day). So unless you're willing to reset all of your AAs and run around in handcrafted gear, you're not really getting an "authentic" experience anyway.