D&D power levels are just as much if not moreso about "stuff" as they are about actual character levels.
The setup you've got effectively increases the power level by quite a few levels, really, what with worlds and equipment tailored to their guys.
The character this guy is making sounds to be effectively lv15 or more.
Even then, I was under the impression that level 6 was closer to the "players start having real power" point. Lv9 or 10 seems to be generally agreed to be the gateway to "high level" play, where the characters are renowned heroes and can take on ferocious monsters on their own, while lv20 or so is the gateway to "epic" play, where they start traversing planes as a matter of course and can probably destroy an entire city on their own if they were in the mood.
If you really want a higher leveled campaign, I'd recommend starting at a lower level but giving experience bonuses up to your "desired" level to give the players some time to ease into their characters.
From some D&D rulebook:
""Heroic" for levels 1-10, "Paragon" for levels 11-20, and "Epic" for levels 21-30."
In my own experience:
1-4 : Adventurer (Low Level) - You are just getting started, but are capable of some pretty impressive feats.
5-9 : Hero (Mid Level) - You are known nationally, maybe beyond, for your exploits.
10-14: Champion (High Level) - Few in this world operate at your level, and you're likely known far and wide for your skill.
15-19: Demi-God (Low-Epic) - You are beyond what a mere mortal can ever dream of becoming, like Hercules, your feats are simply supernatural.
20-24: Ascended (Mid-Epic) - You travel between worlds slaying the most powerful creatures that exist with ease.
25-30: God (Epic-Epic) - No description needed.