- PCs in d20 Modern didn't start off with spell casting. The first few levels were basic classes, and the PCs had to qualify for advanced classes at a minimum of 4th level.
- Normal PC spellcasting is limited to a maximum of 5th-level spells. Spells of 6th level or higher were part of incantations, which anyone could do, but could go wrong with some bad skill checks.
Now, looking at D&D 5e, it's easy to max out the spells available at 5th-level. With level boosting, a magic user could use higher-level slots to cast their lower-level spells with an effective boost.
Trying to put some levels between Level 1 and Magic is tougher. The martial no-magic classes each have a class option to add magic to the character at level three (Barbarian has the Totem Warrior, Fighter has Eldritch Knight, and Rogue has Arcane Trickster).
So the line should be 3rd level, right? What about full casters? A wizard can't just start at 1st character level. It'll have to wait until 3rd. That means the wizard has to multiclass, but the Ability Score Improvements are often at multiples of 4 in class level, meaning a) the wizard, if she starts at 3rd level, would gain the Ability Score Improvement at 6th level, or b) the wizard wants to stick with the first class from level 1 until the first ASI at level 4.
In other words, the wizard has to choose between 2 levels of magic and an ASI.
Also, having the magic-starting level would be a pain with the magic line (where a class can choose a flat-out magic option) for specific classes would be different from class to class. Let's call the character level where a PC can take magic classes X. The outright casters would be at X, half casters at X-1, and third-casters and non-casters at X-2. But then you'd have to move the paladin to X, because Lay on Hands was an obviously magic effect. Then, there's certain things like Bardic Inspiration and any healing ability that could be decoupled from magic and set on its own.
Thing is, I'm looking at this project and I see the classes would have to be remade if I want to have that minimum level at any point other than 1st character level. If I was using d20 Modern classes, I'd aim for 4- or 8- level classes.