More to the point, what are your older participants supposed to do to protect their retirement savings and continue to grow their retirement nest egg?
Having most of the money in bonds may mean an upside of little more than 1% interest and when rates go up again, most bond prices will go down, initially offset by only 1% interest. Bonds may now offer more risk than reward.
In a 2020 target date fund, the bond portion may provide more risk than your participants expect, leaving them with risk assets (stocks) and more risk assets (bonds with falling prices).
It might be time to talk about your investment options.
You might want to talk to Dave Hoshour for ideas.