in sentences and phrases with ‘than’, you should use less with numbers when they are on their own:
√ His weight fell from 18 stone to less than 12.
√ A person with a score of less than 100 will have difficulty obtaining credit.
and when talking about distance, time, ages, and sums of money:
√ Companies less than five years old are the ones bringing us new job creation.
√ Per capita income is reckoned to be less than 50 dollars per year.
√ Heath Square is less thanfour miles away from Dublin city centre.
But hold on, I hear you say – the measurements (years, miles, dollars, etc.) are in the plural, so why isn’t fewer the correct choice? Not so! We use less in such cases because we’re actually still referring to total amounts (of time, money, distance, etc.) rather than individual units.
Nguồn: Oxford words