[war1025]

Did a quick search, and this link [1] from what appears to be a reputable source says most household wells are pulling up water that is less than 10 years old.

Granted that probably varies a lot by region.

I've heard of a fair number of people that have their wells go bad and need to drill deeper to get clean water.

Found this page [2] that lists registered well depths here in Iowa. Seems that somewhere in the 300-400ft range is common. Deeper for municipal wells.

[1] https://agwt.org/content/how-old-your-well-water

[2] https://www.iihr.uiowa.edu/igs/geosam/depth_map