Motoki, one of the talented regulars at the More Awesome Than You forum*, has created a modification ("mod," "hack," whatever) that enables ALL the Maxis Adult clothing to work with Young Adults (if you have University) and Elders.
*To log in without paying the subscription fee, use the password gullible.

What this Mod does and does not do:
• ALL the Maxis Adult clothing ~ male and female ~ that comes with the base game is available to Young Adults and Elders.
• Does not include University, Nightlife, or Open For Business skins at this time.
• There is NO EFFECT on Elder skins; they remain available as well, giving you more choice in Elder body shapes (see comparisons below).
• Elders still stand and walk like Elders, they're just not so droopy.
• Young Adults in Adult-based skins will look the same, just as they do by default.
• Be careful not to mix Adult-based with default Elder Tops and Bottoms; doing so will leave ugly gaps!
• Maxis Adult skins exported and recolored through Body Shop will be available to Adults, Young Adults (if you have Uni) and Elders, even without this file installed ~ if you make skins for download, downloaders don't need this file to use your converted skins (but it's such a neat mod, I hope you'll encourage them to get it!).
• IN ADDITION to everything it does for Young Adults and Elders, this mod ALSO unlocks, and makes available for recoloring, all NPC outfits and pregnancy clothing that comes with the base game! (Does not include EP skins.)
• Also recategorizes various appropriate clothing types for use as GYM CLOTHES.
• Makes Korean Formalwear available as Everyday.

The .package file does not make any changes to the actual Maxis skins, or to any game files. Simply remove the file to uninstall it. Any modified custom skins will remain available to Young Adults and Elders, but Maxis Adult skins will once more be available only to Adults.