What you need:
YouTube Android and iOS app for the related mobiles.
If You are located in the following countries:
India, Indonesia and the Philippines.
Why these Asian countries chosen?
Because the internet speed is much slower.
Is there any time period for viewing offline?
According to Google, videos can be played back without an internet connection for 48 hours once taken offline.
To download videos users can simply tap on the offline icon that sits under the video frame and select the desired quality. The icon will be visible only against videos that have offline viewing enabled.