I feel like they're stretching it with this "10 year difference". I'm in better shape than I ever was as a teenager as I became obese in my first years of college due to depression, just turned 30. After treatment, I became conscious about my health and made an effort to improve. So this whole 10 year difference is weird. Hell, when it comes to cardio usually a few months of practice is all you need, even if he is compromised due to his smoking, it took me close to a year though because of my obesity and asthma combined. In fact, my sister is a heavy smoker and it took her half a year to to get her cardiovascular system to keep up with the work outs she was trying to complete.
So yeah, this 28 year old thing is stretching it, especially when you consider he has been taking gym class for almost 7 months now.