Several parts combine into one character.

代 is a phono-semantic character: 人 (亻, person) for meaning and 弋 (a stake / arrow) for sound. One person "substituting for, replacing" another is its core sense. From people taking turns and succeeding one another grew "generation, era, an age."

Korean hanja-sound "dae": 代身 (daesin, instead), 時代 (sidae, era), 世代 (sedae, generation), 代表 (daepyo, representative), 交代 (gyodae, shift / taking turns), 代理 (daeri, proxy), 現代 (hyeondae, modern times). It is a counter for the "reigns (代)" of a dynasty, and also marks age brackets (20代, 30代).

Mandarin: dài, 4th tone. 代表 (dàibiǎo, representative), 时代 (shídài, era), 现代 (xiàndài, modern), 代替 (dàitì, to replace), 古代 (gǔdài, antiquity), 交代 (jiāodài, to hand over / explain). Used in historical periodization, as in 朝代 (cháodài, dynasty).

Japanese: on-readings ダイ (dai) and タイ (tai), kun か.わる (kawaru), よ (yo), しろ (shiro). 時代 (jidai, era), 代表 (daihyō, representative), 交代 (kōtai, taking turns — a タイ reading), 身代わり (migawari, a substitute), 千代 (chiyo, a thousand ages). Japanese 千代 (chiyo) means "a thousand generations = eternity," appearing in 君が代 (the national anthem).

Memory aid: a person (亻) standing in for another — generations taking turns and succeeding.