Under benevolent rule, both old and young are at peace.
Tips
history
Traces back to Mencius (《孟子·梁惠王下》): 反其旄倪 'return their old and young' - 旄 was later written 耄. 耄 = aged 80+, 倪 = small child, so 耄倪 brackets the whole age range.
register
Strictly literary / classical. You'll see it in historical writing, not conversation.
