"Grandpa Jason and Grandpa Chad", by Randall Munroe, licensed under Creative Commons Attribution-NonCommercial 2.5 License
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The AARP puts the average age of a first-time grandparent close to 50, and the CDC has some data. But I don't have first-parent age distributions for specific names, or generational first-child age correlations, so the dotted line is just a guess. Still, let's be honest: No further research is really needed.