Hairstyle, tattoos/warpaint and scars(provided they're restorable) are an okay.
But not body types. If there was a way to change body types it should be implemented in a way that makes sense but also means that you
choose that body type.
And aging?
Maybe there could be an option for aging but it would be unfair to impose that on anybody.
All of these would be fine as long as the last two were optional and the scars could be removed.
But as much as people argue "I spent hours on my character and I don't want them to change", where's the problem if it is a decidedly optional feature? I spend hours on my characters too and I always end up finding cause to go back and change them. For the most part this would be a godsend. If only there was a lore-friendly explanation for the ability to change facial features, you know, for those little nuances you didn't quite notice in the terrible lighting conditions, such as: oh, I had no idea my character was luminescent; where did that purple smudge come from? why is their neck a very different colour from their body?
If Skyrim has a better character creation tool and better lighting then maybe this wouldn't be a problem to me, but the first two are fine. They make sense, there
were barbers back then, and tattooists. Scars would be fine as long as they were removable. Changing body types could be implemented without making it a Sim-feature, and aging would have to either not be included or be optional.
This idea is so bad that it's only surpassed by the strange idea that your characters apearance somehow change when you do good or bad deeds like in Fable and Mass Effect.
Only happens in ME2 and at that, it's optional.
If your diet consists largely of venison, corn and potatoes and you labor at a sawmill you're going to be pretty lean and muscular most of your life, regardless of how much you eat.
I think you'll find that the key clause there has been emboldened. If you're a wealthy landlord who earns money for telling other people to do all that labour, you're still getting the same diet, just no exercise. That said, for the most part, you are correct. Characters like the player are hardly going to put on weight with the lifestyle they have.