What is the average life expectancy of a US coin?

It depends on the type of coin. Nickels can last five years pennies 1 year dimes 10 years quarters 25 years half dollars 50 years golden dollar 100 years

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I have to disagree, based on years of observing pocket change. If by "life expectancy" you mean the point when coins from a given year start becoming uncommon (however one defines that Word) due to wear/loss/collecting/etc., certainly the figures would be more on the order of

Cents - about 15-20 years

Nickels, dimes, and quarters - about 30-50 years. These coins are made of cupronickel which is very hard; noticeable wear only sets in after 30+ years in circulation.

Halves - It's very difficult to say because they no longer circulate widely due to errant business and government policies

Dollars - The new manganese-brass coins haven't been out long enough to establish a track record, and these as well will not circulate widely until the dollar bill is finally made obsolete.