They are similar, but you can only må good/bad/weird etc., you can’t må old. Må is more if you are in a certain condition, and old is not really a condition.
So må is for an emotion rather than a mental state? Does that mean that, e.g. jag mår trött does not work?
'Emotion' isn't quite right, you can just må better or worse, not happy or something like that. Basically it's about your state of health, though it can apply to your mental health as well as your physical health.
I don't understand the difference between "känner sig" and "kanns". Is "känns" a interpretation of a physical stimuli while "känner sig" is about emotions? ie, one can känns varm or känner sig ledsen, but one can't känns ledsen or känner sig varm?
There's a difference of perspective. känner sig is about how someone feels in the sense what they experience. How något känns is about how the speaker experiences the thing. So jag känner mig varm 'I feel warm', (I experience this), but Rummet känns varmt 'The room feels warm', the room does not experience this, I do.
I still try to figure out the difference between känner and känns: Is "Hur känner du" how you feel (How do you feel?)? And "Hur känns du?" the feelings you have about feeling yourself? "How does it feel to feel yourself?" Is känner - känns a little bit in the way you use our German word "wahrnehmen"?
Have you read my post about the deponent verbs here: https://www.duolingo.com/comment/6094592 ? It might be helpful.
hur känns du would only refer to someone else's experience of you, so it would be somewhat odd to ask that.
If someone touches you and thinks your skin feels cold, they could say Du känns kall 'You feel cold [to me]', because then they experience that.
But you yourself would just känna dig kall.
I think, now I understand a little bit better: It is in a way like in German "fühlen" and "anfühlen", "ich fühle (etwas)" and "Ich fühle mich ... an".
"Ich fühle mich alt" = "Jag känner mig gammal". "Ich fühle mich alt an" = känns. (You wake up one morning, touch your skin, count the years... It is, even in German, an unusual, more philosophical sentence, but it works.) So "Du känns gammal" is a sentence you better shouldn't say to your girlfriend or wife. I hope, I've got it...
Can one say han käns gammal, meaning that you feel like the man must be old? Or would that be streching it?
I guess that "Han känns gammal" works, but it could sound as if you touch the man and find a lot of wrinkles :). If you mean "he seems old" then "han verkar gammal" is better.
In literal translation, would this mean "He knows his age," and thus be figurative language for "He feels old?"
No, the literal translation would be 'He feels himself [to be] old' – where 'old' is an adjective referring to him.
'age' is ålder