"Fewer" is used with countable nouns. Years, days, pigs, dogs, cars, houses, questions. I have a few questions. I have fewer than him. There are fewer dogs in my city. "Less" is used for uncountable nouns. Time, money, energy, etc. I have less time than him, I can do it in less time than you. In theory, less is only used for uncountable nouns, but in practise it's also used for countable nouns too by most people. Also, we generally don't use "fewer" for quantities smaller than one. Even if the one thing is big. Fewer than one is zero. If there is part of something, we use "less than one". In your sentence, "less than" really means "less time than". And it's referring to decades. Less than one decade. Less time than one decade!