(a) His aunt is younger than his uncle. 彼の叔母さんは伯父さんよりも若い。
(b) His uncle is (older) than his aunt.
彼の伯父さんは叔母さんよりも年をとっている。
(c) His uncle is (less) young than this aunt. ※little(原級:…ない)-less(比較級)-least(最上級)
彼の伯父さんは叔母さんよりも若くない。
(d) His uncle isn't (as) (young) (as) his aunt.
彼の伯父さんは叔母さん程若くない。
以上です。