I am 27 now and have only flown in premium cabins a hand full of times, starting at about 8 years old. Most recently flew on my honeymoon in DL F and CX J and was treated the same as everyone else, with no glares or questions.

As a lot of people have touched on I think it also depends on how the child is acting. When I was 8 flying LX J and when I was 13 flying DL Biz Elite, both times the FA's complemented my brother and I on our manners to our parents. (My father lives by the phrase "Please and Thank you are the gateway's to the world"). The only time my now wife and I were questioned was flying ATH-JFK-MCO a few years ago, Ground crew at both ATH and JFK stated boarding was only for DL Biz Elite pax, I quickly stated good, and handed them our BP's.

When I was younger traveling with my parents they would dress me up nicer, but lately on my honeymoon I dressed in just a button up, jeans and flip flops, always have to go for comfort flying those long haul routes