From the age of eight to the age of eighty, a person should fast on all of the Ekadashis on both the light and the dark parts of the month.
ARE THERE ANY EXCEPTIONS?
eka bhuktena naktena bala vrddha aturah ksipet payo mula phalair vapi na nirdvad asiko bhavet (Markandeya Purana)
A child, an old man or a person unable to fast can take something to eat in the evening or once in the day, whether it be milk, fruit or water, but should properly follow the Ekadashi fast. One should not give up fasting on Ekadashi.
According to Hari Bhakti vilasa anyone over the age of 8 should follow ekadashi. But they could eat & drink before sunrise and after sunset as that allowance is called sankalpa.