Best and worst explanations of why immortal characters look older in sequels

I always thought this was an interesting part of film.

When a character is supposed to be immortal or otherwise ageless, but actors have aged just a little too much in later sequels.

For example, in the last Terminator they had to explain how a Terminator could get older and I think said something along the lines of his hair and skin being all organic and so aged with time despite being a machine, correct?

In The Hobbit they just tried to CGI Orlando Bloom's youth back into him (kudos to Hugo Weaving by the way).

I think I first thought of this when I watched The Santa Clause 2 when I was younger. All the elves in the first one were supposed to be hundreds of year old, but were all pre-teens in a matter of a couple of years.