Proportionally they look about the same as Hilbert and Hilda, who were designed to be around 16 but were later decided to be 14. I think Victor and Gloria look about that age too, but maybe that's because of Yusuke Ohmura being the illustrator.
You can find confirmed ages for a number of protagonists.
>Red (and by extension, Leaf): 11, from the Gen I manual
>Brendan/May: 12, from ORAS
>Calem/Serena: 17, from saying they're older than the 16 year old Emma in XY
>Elio/Selene: 11, said multiple times in SM/USUM
>Rei/Akari: 15, estimated age in PLA
But we can go further and find the heights and weights for Lucas/Dawn and Ethan/Lyra and then compare them to the average weights of Japanese boys and girls by age. Lucas and Dawn are significantly shorter and lighter than the Johto protagonists, so it's probably accurate to say there's a year difference.
