молодо́й • (molodój)
"young; youthful; new"
I think that младший (mládšij, "younger") was borrowed from Old Church Slavonic, while молодо́й (molodój, "young") is an inherited East Slavic word, as the latter displays pleophony (полногласие), but I can't find any hard confirmation.
So much new vocabulary! Russian Duo could learn from Czech Duo where the vocab is in the Notes/Tips. Oh, and do not mention the Click a word input. I am angry that Duo recently broke the nexus between the two forms of input! Because you cannot copy-paste your correct answer for future reference, Word Bank is a broken learning tool !