couple with a large age gap
Refers to a romantic couple where the partners have a significant age difference. The phrase is neutral and commonly used in media and everyday conversation.
That couple with a big age gap looks like parent and child, but they really get along well.
Apparently, couples with a large age difference have been increasing lately.
Compound of 年 (とし, 'year, age'), の (possessive particle), 差 (さ, 'difference'), and カップル ('couple'). The exact origin is uncertain, but it likely emerged in modern Japanese as a descriptive phrase for couples with a noticeable age gap.