Starfox
Hero
In my experiences, the game starts to feel like it's on easy mode somewhere around 14th level.
Strategic thinkers will gladly make some sacrifices at level 1 to become stronger at higher levels. If the game is set to accommodate them at level 1 and then uses a "normal" power progression as the default as levels increase, it will fail to accommodate them at higher level.
This has very much to do with build optimization. Basically, it is the same thing that happens in MMOs. As you advance in level, you gain more character choices. Make bad choices, and you slip behind the power curve. Make good, synergistic choices and you slide ahead of the power curve. As some players make good choices throughout their career this gradual change accumulates, with each choice they'll widen the gap. At some level, the gap becomes so great that things are no longer challenging.
Add in the fact that some builds take time to bloom. If character options A, B and C together make a great combo, but having only A and B is suboptimal, a character going this path will seem weaker until he gets C as well and the combo blooms. 4E is full of such combos. I guess in your game they bloom at about level 14. In my game it happened at around level 12, and again at level 22, when characters could exploit the new options at each tier.
The effect of tactical savvy is harder to calculate, but even there, it usually takes certain character abilities to use tactics to the full extent. As players customize their characters to fit the tactics they have mastered, there will be more and more synergies between tactics and character abilities, further escalating things. Players also learn to mesh their characters to the team - a party that has played together since level 1 will be stronger than one that just happens to form at a game-store event, even if the characters individually are just as optimized.