which card should we buy in hong kong?

I'm coming to HK with my mother, we'll stay longer than 14 days, so I know we cannt buy the travel pass. Which tickets would it better to buy to go from and back to the airport? woud the octopus card be convenient? or woud it be more convenient to buy separately the transfer from/to the airport and an octopus to use when we are in Hong kong?
Which are the fares for me (age 62), for her (age 89)?
thanks regards
marina tritapepe