be mutton dressed as lamb
be mutton dressed (up) as ˈlamb(British English, informal, disapproving) used to describe a woman who is trying to look younger than she really is, especially by wearing clothes that are designed for young people: Have you seen her? Mutton dressed as lamb. Somebody should remind her that she’s 55, not 25.
Mutton is the meat from an adult sheep, while lamb is the meat from a young sheep.