"My mom is younger than yours."
Translation:Mi mamá es más joven que la tuya.
I think there are a few things here: tuyo should be tuya instead, since madre is feminine. "Mas" always goes with"que" in these "mas ____ que" phrases. Como would go with tan (mi madre es tan joven como la primavera - my mother is as young as the springtime). And the "de" isn't necessary because the "que" already covers the comparison. It's redundant in this case.
Also it should be "la tuya" instead of just "tuya", because we avoid repeating the same word (mamá) twice. See also https://www.duolingo.com/skill/es/Description-3/tips