noun
young owner; young proprietor
Refers to a young person who owns or runs a business, often a family-run shop or inn. Carries a nuance of youth and inexperience, sometimes with a sense of respect or affection.
あの旅館の若主人はまだ二十代だ。
The young proprietor of that inn is still in his twenties.
若主人が店を継いでから、雰囲気が明るくなった。
Since the young owner took over the shop, the atmosphere has become brighter.
若旦那 is a more traditional term for a young master, often used in a historical or high-class setting, whereas 若主人 is more neutral and modern.
Compound of 若 (young) and 主人 (owner/master). The reading is a straightforward combination of the two words.