IMHO - NO kids NO babies in first or business class period end of story! I guess if you are above a certain age then yes it is acceptable but be that 16 for a drivers license point of view or a 12-14 year old I don't know.
Here's my point - and for context since none of you know me I am not ordinarily an exclusionary type of person however....
Last Friday I was in First Class on a BA 747 LHR-SAN when two seats in first class were filled with Mom, Dad, a toddler and a newborn (as in less than 6 months old). First was full except for one seat. Baby cried yet the toddler threw fits throughout the first 1/3 of the flight before they both fell asleep until landing.
First class is the last respite in a travel world filled with chaos and yes it's a class-ist system but if I can afford to pay for First Class to avoid the chaos or cattle cart and get peace quiet and comfort along with hopefully better service then I will and I won't take it lightly with misbehaved or crying/screaming temper tantrum kids.
So with that NO kids in first class. And when I take my niece of 10 and nephew of 8 with me - the same rule applies to me and I will be back in coach in a middle seat as they like the window and aisle and yes I will be fine with it.
I think airlines should put in policies and rules that help this notion too. There I said it. If you feel the need to flame me then be objective or don't flame me at all.