McKenna is three months younger than Matthew Schaefer. They were both born in 2007. Both players are in the midst of their second full CHL seasons. Thus, they should be directly compared as age-mates.
The mid-September birthday cut-off for the NHL is complete arbitrary and meaningless to player development. The only reason it exists is to ensure that 17 year olds can't play NHL games.
The January 1st age-year cut-off is much more meaningful, as guys have played at various levels of hockey for their whole lives based on this cut-off. For example, Porter Martone is in his third full CHL season, as he is an October 2006 birthdate. Thus, he would be expected to compete with the likes of Dickinson and Catton on this team. His extra year of CHL time prior to being drafted should put him ahead of 2007-born Matthew Schaefer in terms of development. James Hagens is playing his draft year in the NCAA as a natural consequence of being a December 2006 birthdate, having played two USNTDP seasons with his 2006 age group. Celebrini played his draft year in the NCAA as a result of accelerating his schooling and playing in the NCAA at 17, a year ahead of what the rest of his 2006-born age-mates were doing.
Make sense?