A mixed age couple is where one of the couple has reached state pension age and the other has not. Check my state pension age.
If you and your partner have not both reached state pension age, you may only be able to get help with your rent by making a new application for Universal Credit.
New applications for Housing Benefit are only allowed for mixed aged couples if you and your partner were in receipt of Pension Credit before 15 May 2019, and you continue to receive Pension Credit.