I actually don't see a problem with juniors in the lounges, as long as they are well behaved. A 15 year old does not get a discount on his premier ticket. It's better the airline know where their premier passengers are - and BA could create a lounge space for their junior golds and premiers.. At smaller loungers, it is not really an issue as the lounge dragon could enforce the rule

Rather than parents encouraging juniors to break the rules... perhaps the airline should be more accommodating..