When working for BEA in LBG in the 60's/70's as Officer i/c of Load Control, we would use the average weights for all our flts, whatever acrft type : 78 kg for a male, 68 kg for a female and 12 kg for a child up to 12 years of age. Those weights included 5 kg for hand luggage. The baggage wts used were 12 kg per piece. The only times we had to weigh our pax/bags was for the Cambrian Airways flts operated by a DC3 with 32 pax. The reason being that in UK the max allowed TOW was 12700 kg and in France that DC3 had a restricted max TOW of 12200 kg. So of course once every 2 years when the Wales vs France rugby match took place in Paris, you can imagine what we were up to. Unable to legally load the bags on the DC3 return trips to CWL as TOW exceeding 12200 kg. So we topped up the loads on the Viscounts luckily.
I remember a little later having changed Cy and operating as Stn Mgr for a US airline, I requested several times the actual pax/bags wts for a few LBG/JFK flts operated by a DC8-63 252 seat config. sked to depart around 13:00Z in July/August. We gained each time around 2500 kgs that allowed the last minute top up fuel in order to fly non stop. Obviously, in those days the human beings were not as heavy as they are today.
Those were the good days.