Younger people get very few joint replacements, yet they're also getting more than older people did at the same age. This means you can choose between 'Why are millennials getting so (many/few) joint replacements?' depending on which trend fits your current argument better.

"Cohort and Age Effects", by Randall Munroe, licensed under Creative Commons Attribution-NonCommercial 2.5 License

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Younger people get very few joint replacements, yet they're also getting more than older people did at the same age. This means you can choose between 'Why are millennials getting so (many/few) joint replacements?' depending on which trend fits your current argument better.