I mean in Sims 4 Studio. There's no way to import separate meshes for each age group within one package file. I tried to make different swatches with the model's respective age groups checked off, but all the swatches just defaulted to the last model I import. Any idea how to go about with this? Thanks

S4S is only designed for focusing on one item at a time, and by extension clothing items don't typically share a resource between ages anyway. For the ones that share the same mesh, just enabling those age flags is enough.
For the others, you'll want to make each one separately. The packages can be merged under Content Management > Merge Packages when you are done.

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