A magic item's caster level (CL) is not a prerequisite for its creation

Usually only a magic item's prerequisites must be satisfied for the creator to create the magic item; the item's CL is a different, separate part of a magic item's entry, but a magic item's prerequisites often influence the minimum CL that a magic item can have.

The Dungeon Master's Guide (2003) didn't make this very clear, saying on Magic Item Descriptions on Caster Level that

The next item in a notational entry gives the caster level of the item, indicating its relative power (just as a spell’s caster level measures its power). The caster level determines the item’s saving throw bonus, as well as range or other level-dependent aspects of the powers of the item (if variable). It also determines the level that must be contended with should the item come under the effect of a dispel magic spell or similar situation. This information is given in the form “CL x,” where “CL” is an abbreviation for caster level and “x” is an ordinal number representing the caster level itself.

For potions, scrolls, and wands, the creator can set the caster level of an item at any number high enough to cast the stored spell and not higher than her own caster level. For example, at 5th level, Mialee could scribe a scroll of invisibility at caster level 3rd (making it last 3 minutes), caster level 4th (4 minutes), or caster level 5th (5 minutes). For other magic items, the caster level is determined by the item itself. In this case, the creator’s caster level must be as high as the item’s caster level (and prerequisites may effectively put a higher minimum on the creator’s level). (215)

(Strikethrough mine.) Then the Dungeon Master's Guide errata has this entry:

Caster Level

Dungeon Master’s Guide, page 215
Problem: The last two sentences in the section on Caster Level are ambiguous and potentially misleading.
Solution: Replace with this text: For other magic items, the caster level is determined by the creator. The minimum caster level is that which is needed to meet the prerequisites given. (1)

(Italics mine.) This erratum's folded into the two SRDs that I use regularly—here and here—as well as the premium edition of the Dungeon Master's Guide (2012), but this erratum is not present in the Dungeon Master's Guide (2003) (obviously—it's errata!) nor here in what looks like a barely modified reproduction of Wizards of the Coast's original SRD for D&D 3.5e that, so far as I can tell, is offline at its original home here but remains available via the Internet Archive here. So, yeah, some confusion's understandable, and the erratum's author's gift for understatement hasn't gone unnoticed.

Thus, when a creator wants to create the typical magic item, the creator must meet the magic item's prerequisites. Then, prior to starting her creation, the creator sets that magic item's CL to the minimum necessary to meet the magic item's prerequisites. Since many magic items have spells as part of their prerequisites, that's usually the minimum caster level of those spells. (This minimum caster level doesn't seem to apply also to the minimum caster level necessary for meeting the appropriate item creation feat as there are many examples of wands, potions, and wondrous items possessing, for example, CL 1.)

For example, while the typical magic weapon special ability frost (DMG 224) (+1 bonus; 0 lbs.) is printed as having CL 8, the special ability can be added to a magic weapon by a druid that meets the special ability's prerequisite via the 2nd-level Drd spell chill metal [trans] (PH 209), therefore making the special ability's minimum CL 3. A wizard could, instead, meet the special ability's spell prerequisite with the 4th-level Sor/Wiz spell ice storm [evoc] (PH 243), therefore making the special ability's minimum CL 7. Finally, a creature possessing levels in the prestige class disciple of Thrym (Frostburn 56-8) could meet the special ability's spell prerequisite with the spell ice storm, but for that creature the spell ice storm is a 5rd-level spell, therefore making, in this case, the special ability's minimum CL 9.

Some items will have as prerequisites a minimum caster level, and these are actual prerequisites that're mandatory for the item's creation. For example, a ring of protection +2 (DMG 232) (8,000 gp; 0 lbs.) typically has CL 5, but the ring's prerequisite says that such a ring can only be created by a creator of at least a caster level of 6. However, the spell prerequisite—the 1st-level Clr spell shield of faith [abjur] (PH 278)—, remains unchanged, so the creator can set a ring of protection's CL based on that spell, from 1 for a typical cleric to up to the creator's actual caster level.

As can be seen by these examples, designers seem to have set some magic item CLs arbitrarily.

Solving the universal solvent

To be clear, the universal solvent (DMG 268) (50 gp; 0 lbs.) says CL 20 then lists its prerequisites as the feat Craft Wondrous Item and the spell disintegrate. A sorcerer creator of universal solvent could set that CL as low as 12, a wizard creator as low as 11, a cleric that possesses the domain Destruction as low as 13, or a duskblade (Player's Handbook II 19-24) as low as 17. None need wait until a caster level of 20 is attained to create the universal solvent.

Also, readers should be aware that the universal solvent changed significantly during the 3.5 revision. Removed from the description of the revised solvent is the entire reason why it has such an outrageous caster level:

If the liquid is carefully distilled to bring it down to one-third of its original volume, each dose (1/3 ounce, having been a full ounce before distillation) dissolves 1 cubic foot of organic material, just as if a disintegrate spell had been employed. (Dungeon Master's Guide (2000) 227)

(Using the solvent this way requires a touch attack and the subject makes a Fort save (DC 19).) Further, the original description says that even the normal, undistilled version of the older solvent "negates… any other form of cement, glue, or adhesive" rather than only sovereign glue and tanglefoot bags. Finally, the original universal solvent costs 2,000 gp. The 3.5 revision, then, changed nearly everything about the universal solvent except its prerequisites… and the CL 20.