That looks like a lot of fun in PowerPoint...
I am wondering if the age should really be a factor but I guess we can leave it in. Age/Experience does come into play when the two opponents are relatively even physically. Also, it's stereotypical that a younger guy can last longer in an endurance match rather than an elder fighter, but that also depends on the type of training that the older fighter have done over the years. There is a reason why old Asian men are pretty much off the mugging list of any smart criminal...
But I think that overall, physical fitness doesn't really play a part unless the magic and/or their style calls for it, so I really don't think it needs to be factored in as a separate attribute that affects the overall tide of a battle in regards to Stamina. Elfman's pretty buff but he constantly loses out to Natsu and Gray and that's because they apply their magic far differently than he does. Besides, the physical fitness attribute might do better to be slotted under the 'Durability' attribute since a better physique does allow for a stronger, more duarble body.
Natsu will pretty much fight as his opponent fights so if the guy goes bare-handed with no sliver of magical power, Natsu usually just strong-arms them. I'm referring to the first battle against Bora. Natsu didn't use his magic against the mooks, just against Bora. Whether they got caught up in the explosions afterward, that's a different story.