if you're going direct I cannot see why they dont serve to 18-21 upon takeoff. if you are bothered, call CX and ask (call HK, where they wont use their "common sense" in the US). I suppose they would serve the preflight drinks to them too, especially when dad's onboard.
The only time i had problems was on 1 of the HKG-LHR student carrier and they made me show passport (I was 18+ so there was no problems) other than that 1 flight, no problems at all.

Anyway, sometimes upon op-up I do drink far more than I should. (whoops) and I remember once my sister who was 15 then got offered drinks. So I guess if FA think they will be sensible, they'll be offered.