That's because bond funds don't have a stated maturity date and will continue to keep updating the fund will newer bonds as earlier purchases mature. (awealthofcommonsense.com)
Like other bonds, they promise to pay interest on a regular basis and have a stated maturity date when they return par. (blackrockblog.com)
You are also paid the bond's full face amount at its stated maturity date. (us.etrade.com)